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Chapter 13 Flashcards

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1
Q

Mitochondrial and chloroplast proteins are synthesized on

ER ribosomes.

free cytosolic ribosomes.

the Golgi apparatus.

peroxisomes.

A

free cytosolic ribosomes.

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2
Q

Which of the following contain their own genomes?

Mitochondria, but not chloroplasts and peroxisomes

Chloroplasts, but not mitochondria and peroxisomes

Mitochondria and chloroplasts, but not peroxisomes

Mitochondria, chloroplasts, and peroxisomes

A

Mitochondria and chloroplasts, but not peroxisomes

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3
Q

Which statement about mitochondria is true?

They produce most of the ATP derived from the breakdown of lipids and carbohydrates.

They produce all of the ATP derived from the breakdown of fatty acids.

They produce all of the ATP derived from the breakdown of carbohydrates.

They produce about 10% of the ATP derived from the sunlight.

A

They produce most of the ATP derived from the breakdown of lipids and carbohydrates.

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4
Q

The infoldings of the inner mitochondrial membrane are called

cisternae.

cristae.

laminae.

lamellae.

A

cristae.

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5
Q

The inner compartment of mitochondria is called the

stroma.

intermembrane space.

inner membrane space.

matrix.

A

matrix.

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6
Q

Which compound is a product of glycolysis that is transported into the mitochondria?

Pyruvate

Acetate as acetyl CoA

Lactic acid

Citric acid

A

Pyruvate

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7
Q

The citric acid cycle consists of the oxidation of _______ to produce _______.
Question 7 options:

pyruvate; CO2, NADH, and FADH2

acetyl CoA; CO2, NADH, and FADH2

pyruvate; CO2

pyruvate; NADH and FADH2

A

acetyl CoA; CO2, NADH, and FADH2

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8
Q

Most small molecules are permeable across

both mitochondrial membranes.

the inner, but not the outer, mitochondrial membrane.

the outer, but not the inner, mitochondrial membrane.

neither mitochondrial membrane.

A

the outer, but not the inner, mitochondrial membrane.

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9
Q

Mitochondria can

divide by fission.

fuse with one another.

be transported to areas of high energy use.

All of the above

A

all of above

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10
Q

The mitochondrial outer membrane contains channels composed of proteins called

porins.

aquaporins.

connexins.

claudins.

A

porins

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11
Q

The outer mitochondrial membrane contains proteins that

synthesize ATP.

pump protons.

transport pyruvate and fatty acids.

direct translocation.

A

direct translocation.

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12
Q

Electron transport occurs in the mitochondrial

outer membrane.

intermembrane space.

inner membrane.

matrix.

A

inner membrane.

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13
Q

The process by which mitochondria are thought to have arisen during evolution is called

symbiosis.

phagocytosis.

endosymbiosis.

pinocytosis.

A

endosymbiosis.

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14
Q

Most mitochondrial genomes consist of

a single linear DNA molecule.

several linear DNA molecules.

several circular DNA molecules.

a single circular DNA molecule.

A

several circular DNA molecules.

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15
Q

The organisms most similar to mitochondria are

progenotes.

α-proteobacteria.

cyanobacteria.

purple sulfur bacteria.

A

α-proteobacteria.

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16
Q

Mitochondria contain

no genes of their own.

genes for mitochondrial proteins.

genes for mitochondrial proteins and rRNAs.

genes for mitochondrial proteins, rRNAs, and tRNAs.

A

genes for mitochondrial proteins, rRNAs, and tRNAs.

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17
Q

In what way does the mitochondrial genetic code differ from the “universal” genetic code?

Some codons code for different amino acids.

There are no stop codons.

It accommodates less wobble.

All of the above

A

Some codons code for different amino acids.

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18
Q

The proteins encoded by the human mitochondrial genome function in

mitochondrial ribosomes.

mitochondrial DNA polymerases.

respiratory complexes and oxidative phosphorylation.

helicases.

A

respiratory complexes and oxidative phosphorylation.

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19
Q

Mitochondrial DNA is inherited by means of

maternal transmission.

paternal transmission.

random assortment.

Mendelian genetics.

A

maternal transmission.

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20
Q

which of the following is a mitochondrial disease?

Lou Gehrig’s disease

Retinoblastoma

Leber’s hereditary optic neuropathy

Crohn’s disease

A

Leber’s hereditary optic neuropathy

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21
Q

Most mitochondrial proteins are synthesized on

mitochondrial ribosomes from nuclear mRNAs.

cytoplasmic ribosomes; they are imported co-translationally as they are being synthesized.

cytoplasmic ribosomes; they are imported after they are completely synthesized.

mitochondrial ribosomes from mitochondrial mRNAs.

A

cytoplasmic ribosomes; they are imported after they are completely synthesized.

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22
Q

Mitochondrial targeting presequences usually consist of a

hydrophobic α helix.

hydrophobic random chain.

negatively charged α helix.

positively charged α helix.

A

positively charged α helix.

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23
Q

Tim and Tom are

twin brothers with the same mitochondrial disease.

chaperones.

protein translocators in mitochondrial membranes.

transporters of small molecules across the mitochondrial membranes.

A

protein translocators in mitochondrial membranes.

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24
Q

The mitochondrial protein presequence is cleaved off by a protease called

signal peptidase.

presequence protease.

ubiquitin-targeted protease.

matrix processing peptidase

A

matrix processing peptidase

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25
Q

Mitochondrial inner membrane single-pass transmembrane proteins are inserted into the inner membrane

through a transporter called Oxa translocase.

through Tim.

through Tim and Tom.

directly from the matrix into the lipid bilayer.

A

through a transporter called Oxa translocase.

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26
Q

Most mitochondrial phospholipids are synthesized in the

mitochondrial matrix.

mitochondrial intermembrane space.

ER.

Golgi apparatus.

A

ER.

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27
Q

Which of the following is not involved in the initial transport of a protein across the mitochondrial outer membrane from the cytosol?

Hsp70

Tim23

Tom

ATP

A

Tim23

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28
Q

Which of the following is not involved in targeting a protein from the intermembrane space into the inner mitochondrial membrane?

Tim9

Tim10

Tim22

Tom

A

Tom

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29
Q

The import of mitochondrial matrix proteins from the cytoplasm requires

an electrochemical gradient across the inner membrane.

a potassium gradient across the inner membrane.

ATP and a proton gradient across the inner membrane.

ATP and a potassium gradient across the inner membrane

A

ATP and a proton gradient across the inner membrane.

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30
Q

The energy required to drive the transport of small molecules into and out of mitochondria is provided by the

chemiosmotic gradient.

hydrolysis of ATP.

electrochemical gradient.

negative charge of small molecules.

A

hydrolysis of ATP.

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31
Q

Which of the following phospholipids contains four fatty acid chains and is associated with a restriction in proton flow?

Phosphatidylcholine

Cholesterol

Sphingolipid

Cardiolipin

A

Cardiolipin

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32
Q

Chloroplasts differ from mitochondria in that chloroplasts

do not generate ATP.

do not originate by endosymbiosis.

do not replicate by division.

synthesize their own amino acids and fatty acids.

A

do not generate ATP.

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33
Q

Chloroplasts are similar to mitochondria in that both

have a porous outer membrane.

contain light sensitive pigments.

require presequence amino acids on proteins for import.

require folded cristae as the site of electron transport.

A

have a porous outer membrane.

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34
Q

Thylakoids are often arranged in stacks called

dictyosomes.

grana.

plastids.

stroma.

A

grana

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35
Q

Chloroplasts synthesize

elaioplasts.

amino acids.

peroxisomes.

catalase.

A

amino acids.

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36
Q

The chloroplast genome contains about _______ genes.

20

150

1,500

3,000

A

a

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37
Q

Chloroplast tRNAs translate

all mRNA codons according to the universal code.

all the amino acids according to the universal code but have different stop codons.

some codons as amino acids that differ from the universal code but use the same stop codons.

some codons as amino acids that differ from the universal code and use some different stop codons.

A

all mRNA codons according to the universal code.

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38
Q

Most chloroplast proteins are synthesized on

free ribosomes in the cytosol.

RER membranes in the cytoplasm.

ribosomes bound to the outer chloroplast membrane.

ribosomes in the chloroplast stroma.

A

free ribosomes in the cytosol.

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39
Q

The most abundant protein on Earth is

cytochrome c.

rubisco.

ATP synthase.

glucose-6-phosphatase.

A

rubisco

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40
Q

The transport of proteins across the outer and inner chloroplast membranes occurs through complexes called

Tim and Tom.

Tic and Toc.

Sec and Tat.

import complexes.

A

Tic and Toc.

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41
Q

Many proteins incorporated into the thylakoid lumen are synthesized

in the thylakoid lumen.

in the stroma and transported across the thylakoid membrane.

on the outer chloroplast membrane and transported across the thylakoid membrane by way of its hydrophobic signal sequence.

in the cytosol, imported into the stroma, and transported across the thylakoid membrane by way of a second signal sequence.

A

in the cytosol, imported into the stroma, and transported across the thylakoid membrane by way of a second signal sequence.

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42
Q

Carotenoids are stored in

chloroplasts.

chromoplasts.

amyloplasts.

elaioplasts.

A

chromoplasts

43
Q

All plastids, including chloroplasts, develop from

chromoplasts.

etioplasts.

elaioplasts.

proplastids.

A

proplastids

44
Q

An amyloplast is a plastid that

stores starch.

stores lipid.

is arrested in chloroplast development by lack of light.

stores pigment.

A

stores starch.

45
Q

The major function of peroxisomes is to

oxidize certain organic molecules and degrade the H2O2 produced by these reactions.

produce hydrogen peroxide for cells.

digest old organelles.

digest macromolecules taken up by endocytosis.

A

oxidize certain organic molecules and degrade the H2O2 produced by these reactions.

46
Q

Peroxisomes contain the enzyme _______, which breaks down H2O2 into H2O and oxygen.

peroxidase

catalase

peroxigen

glyoxylate

A

catalase

47
Q

Which of the following is the human disease caused by mutations in the proteins required for importing functional proteins into peroxisomes?

Turner’s syndrome

Zellweger syndrome

I-cell disease

Leber’s hereditary optic neuropathy

A

Zellweger syndrome

48
Q

Most peroxisomal proteins are synthesized on

free ribosomes in the cytosol.

RER membranes in the cytoplasm.

ribosomes bound to the outer peroxisome membrane.

ribosomes inside the peroxisome

A

free ribosomes in the cytosol.

49
Q

New peroxisomes form by

budding from the Golgi apparatus.

budding from preexisting peroxisomes.

de novo assembly from proteins synthesized in the cytosol.

budding of vesicles from the ER and growth and division of preexisting peroxisomes.

A

budding of vesicles from the ER and growth and division of preexisting peroxisomes.

50
Q

The citric acid cycle produces two reduced cofactors, _______ and ______

A

NADH, and FADH2

51
Q

To enter the mitochondria, precursors of the β-barrel proteins pass through the _______ complex into the intermembrane space; there, they are recognized and transported to a second complex, called the _______complex, and inserted into the outer membrane.

A

Tom complex

SAM complex

52
Q

All mitochondria use the universal genetic code.

A

f

53
Q

Most mitochondrial proteins are coded for by the mitochondrial genome.

A

t

54
Q

Mitochondria lacking mitochondrial DNA are soon unable to make ATP by oxidative phosphorylation.

A

t

55
Q

Mitochondria contain about 1,500 different proteins.

A

t

56
Q

Most proteins are imported into mitochondria in their fully folded state.

A

f

57
Q

A mitochondrial matrix protein is maintained in its unfolded state by an Hsp70 chaperone protein before transport across the mitochondrial membrane.

A

t

58
Q

Some proteins destined for insertion into the inner membrane are transported from the matrix compartment.

A

t

59
Q

Mitochondrial phospholipids are delivered to mitochondria by phospholipid transfer proteins.

A

t

60
Q

In mitochondria, the pH of the matrix is higher than the pH of the intermembrane space.

A

t

61
Q

Chloroplasts are only one of a much larger family of plant organelles called plastids that all contain the same genome.

A

t

62
Q

Where in mitochondria does each of the following processes or events occur?
Synthesis of mitochondrial ribosomal RNAs

A

matrix
mitochondrial rRNAs are encoded by mt DNA

whereas ribosomal proteins are encoded in the nuclear genome, they are synthesized on cytoplasmic ribosomes, and then are imported into the mitochondria

63
Q

Where in mitochondria does each of the following processes or events occur?
Diffusion of small molecules via porin-formed channels

A

outer membrane

64
Q

Where in mitochondria does each of the following processes or events occur?
The citric acid cycle

A

matrix

65
Q

Where in mitochondria does each of the following processes or events occur?
Oxidative phosphorylation of ATP

A

inner membrane

66
Q

What are some of the reasons evolutionary biologists believe mitochondria came from bacteria through endosymbiosis?

A

Free-living α-proteobacteria have genomes
similar to mitochondria.
Have circular DNA molecules, ribosomes, and transcription machinery thus semi autonomous
double membrane
can replicate on their own

Both generate metabolic energy
• Both evolved by endosymbiosis
• Both contain their own genetic
systems
• Both replicate by division
67
Q

What would be the effect of a mutation in the Tim23 complex?

A

It would be impossible to import mitochondria proteins with presequence from the outer membrane and translocase into the inner membrane into the matrix. It has receptors for the presequences that bind to it and Hs70 chaperons forming a complex

68
Q

Where does the energy for phosphate transfer across the mitochondrial inner membrane to the matrix come from?

A

proton concentration gradient.

69
Q

Both mitochondria and chloroplasts use a proton gradient to drive ATP synthesis, but in different locations. What is the major structural difference, that is related to the proton gradient, between mitochondria and chloroplasts, and where is ATP generated in each?

A

mitochondria it occurs in the inner membrane. The inner membrane in mitochondria is folded into cristae
Chloroplast it occurs in the Thylakoid membrane. , chloroplast contains accessory pigments in thylakoids, which form grana and a stroma. Electron transport and ATP generation
take place in the thylakoid membrane.
Protons are pumped across this
membrane from the stroma to the
thylakoid lumen

70
Q

What effect would a functional mutation in the Toc complex have on chloroplasts?

A

Toc complex a translocase located on the outer membrane used to import proteins into the chloroplast and hydrolyze GTP to GDP for energy for translocation

71
Q

Translocase proteins such as Toc have channels that are just barely wider than a single amino acid. Since most mature proteins have significant secondary and tertiary structure, how do they fit through the narrow channels of these translocases?

A

Hsp70 chaperons keep polypeptide from folding thus keeping the protein in the primary structure

72
Q

What are the two mechanisms by which peroxisomes can be formed?

A

vessicle budding from the ER

growth and division of existing peroxisomes which is a more rapid process

73
Q

Mitochondria differ from other organelles such as lysosomes and the Golgi apparatus in that they

contain enzymes specific to their function.

contain their own genomes.

do not contain proteins that are imported from the cytosol.

are not membrane-bounded.

A

contain their own genomes.

74
Q

What is the major site of energy production in the form of ATP in human cells?

The mitochondrial matrix

The cytoplasm

The outer mitochondrial membrane

The inner mitochondrial membrane

A

The inner mitochondrial membrane

75
Q

The human mitochondrial genome encodes only 22 tRNAs. This limited array of tRNAs can read the 64 possible triplet codons through extreme wobble in base pairing at the third codon position and the use of a(n) _______ genetic code.

chloroplast-mitochondrial-specific

nonuniversal mitochondrial

peroxisomal-mitochondrial-specific

universal

A

nonuniversal mitochondrial

76
Q

Human diseases caused by mutations in mitochondrial genomes

are inherited from both parents.

are inherited from the father.

are inherited from the mother.

do not exist, because the mutation is always complemented by the normal gene copy in the nucleus.

A

are inherited from the mother.

77
Q

Assuming that human mitochondria contain about 1,500 different proteins, approximately what percentage of the mitochondrial proteome is encoded by mitochondrial DNA?

  1. 05%
  2. 1%
  3. 5%
  4. 0%
A

1.0%

78
Q

Which of the following is not involved in protein transport into mitochondria?

A positively charged presequence of 15–55 amino acids located at the N-terminus

The Tom complex

The proton gradient across the inner mitochondrial membrane

Vesicular transport

A

Vesicular transport

79
Q

Which of the following is not a protein translocon/translocase found in the mitochondrial inner or outer membrane?

Oxa1

Tim23

Tom40

Toc75

A

Toc75

80
Q

Where do phospholipids, such as phosphatidylcholine and phosphatidylethanolamine in mitochondrial membranes, originate?

In the ER

In the intermembrane space

On the cytosolic side of the outer membrane

On the lumenal side of the inner membrane

A

In the ER

81
Q

The amino terminal presequence on proteins imported into the mitochondrial inner membrane is

cleaved by mitochondrial matrix processing peptidase (MPP).

transported back to the intermembrane space by Hsp70 chaperones.

cleaved and translocated laterally into the inner membrane.

cleaved during transit from Tom to Tim23.

A

cleaved by mitochondrial matrix processing peptidase (MPP).

82
Q

If a drug blocked the activity of the mitochondrial inner membrane proteins Tim9 and Tim10, which of the following would most likely not be found in the outer mitochondrial membrane?

SAM complex

α-helix membrane proteins

β-barrel proteins

Tom

A

β-barrel proteins

83
Q

A deficiency in cardiolipin would lead to

increased proton flow across the inner mitochondrial membrane.

decreased proton flow across the inner mitochondrial membrane.

increased proton flow across the outer mitochondrial membrane.

decreased proton flow across the outer mitochondrial membrane.

A

increased proton flow across the inner mitochondrial membrane.

84
Q

In terms of its role in the generation of metabolic energy, the inner membrane in mitochondria is equivalent to which component in chloroplasts?

The inner membrane

The thylakoid membrane

The outer membrane

The stroma

A

The thylakoid membrane

85
Q

Chloroplast genomes contain approximately how many genes?

None

40

150

80,000–100,000

A

150

86
Q

_______ different translocon systems are used for protein import from the chloroplast stroma into the thylakoid lumen or membrane.

Two

Three

Five

Ten

A

Three

Tic Toc Alb3

87
Q

The carotenoids, which give many plants their yellow, orange, and red colors, are located in

vacuoles.

etioplasts.

leucoplasts.

chromoplasts.

A

chromoplasts

88
Q

Which of the following plastids is a precursor to all other plastids?

Chromoplast

Proplastid

Etioplast

Amyloplast

A

Proplastid

89
Q

Which factor most likely triggers lipid-containing etioplasts to develop into chloroplasts?

Exposure to light

Hormonal signaling

Exposure to darkness

Internal pigments

A

Exposure to light

90
Q

Peroxisomes are not involved in

the biosynthesis of plasmalogens.

the biosynthesis of the enzyme catalase, which breaks down hydrogen peroxide.

the biosynthesis of lipids.

oxidative reactions leading to the production of hydrogen peroxide.

A

the biosynthesis of the enzyme catalase, which breaks down hydrogen peroxide.

91
Q

Zellweger syndrome is caused by a defect in

mitochondrial protein import.

the electron transport system in mitochondria.

protein import into peroxisomes.

the synthesis of peroxisomal proteins.

A

protein import into peroxisomes.

92
Q

What are porins?

A

The mitochondrial outer membrane contains channels composed of protein

Porins are transmembrane proteins that form large pores and are found in chloroplast and mitochondrial outer membranes, as well as in the outer membranes of Gram-negative bacteria. They allow the passage of molecules smaller than 6,000 daltons, and thus the space between the inner and outer membranes is equivalent to the cytosol in its concentration of ions and small molecules. The inner membrane does not contain porins, and thus ions and small molecules can cross it only in a regulated way via channels and transporters. Porins are also found in the outer membranes of bacteria.

93
Q

Mitochondrial mRNAs have short poly-A sequences at their 3′ end. Poly-A tails are generally considered to be a feature of eukaryotic, not bacterial, mRNA. How can this observation be reconciled with an endosymbiotic origin for mitochondria?

A

The mitochondria of today are not the mitochondria of endosymbiotic origin. Present-day mitochondria have evolved from their original status of newly introduced endosymbionts. In reality, the mitochondrial genome shows a mix of bacterial and eukaryotic traits, with the poly-A sequence a eukaryotic-like trait of mitochondrial mRNA. Presumably this trait originated after the introduction of mitochondria into eukaryotic cells.

94
Q

At low pH, the chemical 2,4-dinitrophenol (DNP) is neutral and can diffuse freely across membranes, including those of mitochondria. At high pH, it gives off a proton, becomes negatively charged, and can no longer diffuse across membranes. What effect would DNP have on the proton gradient between the mitochondrial intermembrane space and matrix?

A

DNP would have the effect of dissipating the proton gradient across the inner membrane. It would be neutral at pH 7 in the intermembrane space and thus pass freely into the mitochondrial lumen, where it would encounter a high pH environment and release its proton. This would neutralize the proton gradient and drastically decrease the overall ATP production of the cell as well.

95
Q

Explain the role of endosymbiosis in the evolution of mitochondria and chloroplasts.

A

Because of the prokaryotic nature of the organelle and the striking similarity between the genomes of mitochondria and some bacteria (most notably Rickettsia prowazekii), it has been hypothesized that mitochondria evolved from an endocytic event in which a bacterium was endocytosed by a eukaryotic cell. The eukaryotic cell would have provided the bacterium with protection from the outside world, and the eukaryotic cell would have benefited from the bacterium’s oxidative phosphorylation system for energy production. The evolution of chloroplasts is hypothesized to have occurred in much the same way but at a somewhat later date.

96
Q

Explain the functions of mitochondrial matrix processing peptidase (MPP) and chloroplast stromal processing peptidase (SPP).

A

First, remember that a peptidase cleaves peptide bonds in polypeptides and proteins. But in both mitochondrial and chloroplast transport there are N-terminal amino acid sequences that target polypepides and proteins to the Tom and Toc complexes in mitochondria and chloroplast outer membrane and subsequently to the Tim and Tic complexes of the inner membranes. In mitochondria, this is the 15-55 amino acid N-terminal presequences, and in chloroplasts, it is the 30-100 amino acid N-terminal transit peptides. Once in the inner membrane, MMP cleaves the presequence in mitochondria and SPP cleaves the transit peptide in chloroplasts

97
Q

Consider two unlabeled centrifuge tubes, each with a pellet at the bottom. One tube contains a pellet of mitochondria and the other a pellet of chloroplasts. How could the pellets be identified?

A

Chloroplasts are full of chlorophyll and thus are green, whereas mitochondria are colorless.

98
Q

Presumably, the original chloroplast genome coded for many more proteins than it does today. Some of these genes must have been transferred to the nucleus, with the resulting protein product needing to be imported into chloroplasts. Other genes must have originated in the nucleus and were then adapted for import into chloroplasts. What were the likely protein adaptations that must have occurred for protein import into chloroplasts, and how might one determine whether the protein originated from a chloroplast or a nuclear-coded gene?

A

What were the likely protein adaptations that must have occurred for protein import into chloroplasts,
Import of proteins into the chloroplast stroma
N-terminal sequences (transit peptides) that direct translocation across the two membranes and are removed by proteolytic cleavage in the stroma by the stroma processing peptidase.
Hsp70 chaperons to keep the protein unfolded to enter both the Tic and Toc complexes on the inner and outer membranes respectively. And where ATP hydrolysis occurs for energy to enter the Toc complex.
Toc complex on outer membrane that hydrolyze GTP to GDP for translocation energy
Tic complex on inner membrane where the Hsp93 binds to its receptors.
Import of proteins into the thylakoid lumen
Two pathways
Sec pathway which is ATP dependent. the proteins have a signal sequence recognized by SecA where ATP hydrolyses occurs to translocate unfolded protein (due to Hsp70 chaperons). Thylakoid Processing Protease (TPP) cleaves the signal sequence in the thylakoid lumen
Twin-Arginine Translocation pathway. The folded protein has a Twin Arginine signal sequence. Translocated via proton gradient energy. Thylakoid Processing Protease (TPP) cleaves the signal sequence in the thylakoid lumen
and how might one determine whether the protein originated from a chloroplast or a nuclear-coded gene?
proteins are synthesized on free ribosomes and imported into chloroplasts as completed polypeptides. And has all or some of the modifications mentioned

99
Q

All plant plastids contain the same genome as chloroplasts. However, chromoplasts, amyloplasts, and elaioplasts are clearly different from one another. What mechanism might explain the differences between these plastids that all have the same internal genes?

A

Plastids are classified based on the
pigments they contain.

development is controlled by environment and intrinsic developmental signals

100
Q

Zellweger syndrome is caused by defects in genes coding for peroxisomal protein import. Why are defects in such genes more likely to be lethal than a defect in a gene encoding a single enzyme present in the peroxisomal lumen?

A 
Mutations that completely destroy Pex
protein function result in severe
disease; mutations that only reduce
function of the mutated Pex protein
cause less severe forms.
101
Q

Zellweger syndrome is caused by defects in genes coding for peroxisomal protein import. Why are defects in such genes more likely to be lethal than a defect in a gene encoding a single enzyme present in the peroxisomal lumen?

A

A defect in import machinery affects the import of several proteins into the lumen of the peroxisome. A mutation in a single peroxisomal enzyme affects only that enzyme. Therefore, the import machinery mutation should have a greater effect and likelihood of being lethal.

102
Q

Presumably the original chloroplast genome coded for many more proteins than it does today. Some of these genes must have been transferred to the nucleus, with the resulting protein product imported into chloroplasts. Other genes must have originated in the nucleus and were then adapted for import into chloroplasts. In general, protein adaptations must have occurred for protein import into chloroplasts. What were these likely adaptations, and how could one tell whether the protein originated from a chloroplast or nuclear-coded gene?

A

In general, cytosolically synthesized and nuclearly coded protein must have an N-terminal transit peptide that can be recognized by the chloroplast guidance complex. If it does not, the polypeptide will not be guided toward the translocon located in the outer chloroplast membrane. This sequence would need to be added by some kind of gene fusion. Machinery for recognition and translocation would need to be adapted in the outer chloroplast membrane. Homology comparisons could be done to determine whether the protein is more similar to a bacterial or a eukaryotic protein. If it is more similar to a bacterial protein, it presumably originated in the chloroplast, irrespective of where the coding sequence resides today.

103
Q

All plant plastids contain the same genome as chloroplasts. However, chromoplasts, amyloplasts, and elaioplasts are clearly different from one another. How can these differences between plastids that all have the same internal genes be explained mechanistically?

A

All plastid development is under the coordinate control of genes within both the plastid and the nuclear genomes. Amyloplasts and elaioplasts are forms of leucoplasts. Leucoplasts are found in nonphotosynthetic tissues. Nonphotosynthetic tissues (e.g., roots) often have little, if any, illumination by sunlight, so sunlight could be used as a cue for chloroplast gene activation. However, in the case of chromoplasts in flowers, sunlight as a gene expression cue does not provide an obvious explanation. Therefore, there must be subtle positional cues to regulate chloroplast gene expression.

Cell Biology (7 decks)

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