Chapter 12

Question 1

Types of gene expression in bacteria

Answer 1

• Constitutive transcription
• Regulated Transcription
• Post-transcriptional regulation

Question 2

Constitutive transcription

Answer 2

Essentially constant expression of genes that are needed continuously (housekeeping genes)

Question 3

Regulated transcription

Answer 3

Expression that occurs under certain conditions (when there is an environmental change or a lack of a crucial enzyme; requires DNA-protein interactions)

Question 4

Posttranscriptional regulation

Answer 4

After mRNA is synthesized, its abundance can be modified in various ways to influence the amount of protein that is translated

Question 5

Negative control: Repressors and inducers and corepressors

Answer 5

• In the absence of an inducer the repressor blocks transcription
• the inducer causes an allosteric change in the repressor, causing it to release from DNA and allow transcription
• with corepressor present, the repressor blocks transcription

Question 6

Allostery

Answer 6

When interaction between proteins change their conformation and function

Question 7

Positive control: activators and effectors

Answer 7

• Without effector, no transcription even in presence of activator
• with effector, transcription is activated

Question 8

Positive control: activator and inhibitor

Answer 8

• With inhibitor, no transcription - binds to activator preventing function
• Without inhibitor, activator can bind and transcription is activated

Question 9

Lac Operon

Answer 9

• Wildtype function: a repressor binds when inducer is absent and prevent transcription
• Prevents synthesis of enzymes that are not needed

Question 10

Beta-galactosidase

Answer 10

• Breaks the beta-galactosidase linkage of lactose to produce glucose and galactose
• Can also convert lactose to allolactose which acts as an induced compound

Question 11

Permease

Answer 11

Imports lactose

Question 12

Operon

Answer 12

A cluster of genes undergoing coordinated transcriptional regulation by a shard regulatory region

Question 13

lacl

Answer 13

a regulatory gene that is adjacent to but not part of the lac operon

Question 14

lac operon

Answer 14

Inducible polycistronic mRNA
- includes regulatory region, lacZ (beta-galactosidase gene), lacY (permease gene), lacA (transacetylase gene)

Question 15

Lac operon promotor region

Answer 15

• CAP binding site
• lacP (promoter sequence): RNA polymerase binds to the promoter sequence
• lacO (operator): lacl repressor binds to the operator sequence and blocks transcription of the lac operon

Question 16

Lactose unavailable, glucose available

Answer 16

• lacl transcribes mRNA for repressor protein
• repressor protein binds to operator (lacO) inhibiting transcription

Question 17

Lactose and glucose available

Answer 17

• Allolactose acts an inducer to inhibit repressor protein
• lac operon is transcribed at a basal level, which leads to allolactose production for lactose

Question 18

Cis-regulatory elements

Answer 18

Only influence transcription of genes to which they are physically connected on the same chromosome

Question 19

Trans-Regulatory elements

Answer 19

Influence transcription via diffusible proteins and do not have to be on the same chromosome

Question 20

Repressor mutation

Answer 20

prevents repressor from binding to operator; leads to constitutive expression, even in the absence of the inducer

Question 21

Operator constitutive mutation

Answer 21

Prevents repressor from binding; leads to constitutive expression, even in the absence of the inducer

Question 22

Super repressor mutation

Answer 22

Prevents inducer from suppressing repressor; transcription repressed even in presence of the inducer

Question 23

Catabolite repression by glucose

Answer 23

In the presence of cAMP, CAP binds the promoter and increases RNA polymerase activity
- Catabolites produced by breakdown of glucose prevent cAMP production
- cAMP binds to CAP, complex then binds to chromosome enhancing RNA polymerase binding
- Low glucose, lots of cAMP = higher expression
- high glucose, low cAMP = lower expression

Question 24

trp operon

Answer 24

polycistronic and includes 5 gens that work together to synthesize the amino acid tryptophan
- trpE, trpD, trpC, trpB, trpA
- the order of these genes corresponds to sequential steps of tryptophan synthesis

Question 25

trp operon repressor and co-repressor

Answer 25

• Tryptophan acts as a co-repressor
• repressor is activated by the corepressor and binds to trpO to block gene expression
• level of expression influenced by the level of tryptophan (attenuated)

Question 26

trp operon attenuation

Answer 26

In prokaryotes translation and transcription occur in same space
- during early transcription, the mRNA of the trp operon forms multiple secondary structures that influence if transcription continues or not
- if secondary structure forms between regions 3 and 4 trpL is transcribed by the rest is not
- if between 2 and 3 the rest of the operon is transcribed

Question 27

What influences which secondary structure forms

Answer 27

• Slow translation of trpL due to a lack of tryptophan causes the hairpin which allows the continuation of transcription
• Rapid translation of trpL due to lots of tryptophan causes other structure which leads to termination

Question 28

Alternative sigma factors for stress responses

Answer 28

• Normally (37 degrees) a subunit of the RNA polymerase holoenzyme call sigma 70 is translated and incorporated into the holoenzyme. Together they drive expression of a specific subset of genes
• At high temperatures (42) a difference sigma factor is translated and is incorporated into the RNA polymerase holoenzyme, and drives expression of another specific subset of genes

Question 29

Riboswitches

Answer 29

• riboswitches in mRNA can cause inhibition of translation of transcripts involved with thiamin production
• When thiamin is low, translation of the operon continues: when thiamin is high, riboswitches cause ribosomal recognition sequences and the start codon to be inaccessible to 16S rRNA and other translation machinery
• Riboswitches can also affect transcription and RNA stability