Chapter 8 (2)

Question 1

Positive control of transcription

Answer 1

Positive control: regulator protein activates binding of RNA polymerase
-Maltose activator protein cannot bind to DNA unless it first binds maltose
Activator proteins bind specifically to certain DNA sequence
-called activator-binding site

Question 2

Positive control of transcription: Promoters

Answer 2

Promoters of positively controlled operons only bind RNA polymerase
Activator protein helps RNA polymerase recognize promoter
-may cause change in DNA structure
-may interact directly with RNA polymerase
Activator-binding site may be close to promoter or 100’s bp away

Question 3

Positive control of transcription: genes for maltose

Answer 3

Geens for maltose spread out over chromosome in several operons.
-each operon has activator-binding site
-multiple operons controlled by the same regulatory protein called a REGULON
Regulons also exist for negatively controlled systems.

Question 4

Global control and the lac operon

Answer 4

Global control systems: regulate expression of many different genes simultaneously
E.G. 1 - Catabolite repression
-synthesis of unrelated catabolic enzymes repressed if glucose is present in growth medium
-lac operon under control of catabolite repression
-ensures best carbon and energy source used first
Diauxic growth: two exponential growth phases

Question 5

Global control and the lac operon: Cyclic CRP

Answer 5

In catabolite repression, transcription controlled by an activator protein and is a form of positive control.
Cyclic AMP receptor protein (CRP) is the activator protein.

Question 6

Global control and the lac operon: Cyclic AMP

Answer 6

Key molecule in many metabolic control systems
-derived from nucleic acid precursor
-regulatory nucleotide
Glucose inhibits synthesis of cyclic AMP

Question 7

Global control and the lac operon: catabolic operons

Answer 7

Dozens of catabolic operons affected by catabolite repression
-enzymes for degrading lactose, maltose and other common carbon sources
Flagellar genes also controlled by catabolite repression
-no need to swim in search of nutrients

Question 8

Control of transcription in Archaea

Answer 8

Archaea use DNA-binding proteins to control transcription.

-more closely resembles control by bacteria than archaea

Question 9

Two-component regulatory systems

Answer 9

Prokaryotes regulate cellular metabolism in response to environmental fluctuations

1. external signal transmitted directly to target v
2. external signal detected by sensor and transmitted to regulatory machinery (signal transduction)
   - most signal transduction systems are two- component regulatory systems

Question 10

Two-component regulatory systems: Proteins

Answer 10

Sensor kinase: (in cytoplasmic membrane) detects environmental signal and autophosphorylates
Response regulator: (in cytoplasm) DNA-binding protein that regulates transcription
Has feedback loop
-terminates signal

Question 11

Two-component regulatory systems: 50 different systems

Answer 11

50 different systems in E. coli
-phosphate assimilation, nitrogen metabolism and osmotic pressure response
Some signal transduction systems have multiple regulatory elements
Some archaea also have two-component regulatory systems

Question 12

Regulation of chemotaxis

Answer 12

Modified two-component system used in chemotaxis to:
-sense temporal changes in attractants or repellents
-regulate flagellar response
Three main steps:
1. response to signal
2. controlling flagellar rotation
3. adaptation

Question 13

Regulation of chemotaxis: Step 1

Answer 13

Response to signal

• sensory proteins in cytoplasmic membrane sense attractants and repellents
• Methyl-accepting chemotaxis proteins (MCP’s)
• bind attractant or repellent and initiate flagellar rotation

Question 14

Regulation of chemotaxis: Step 2

Answer 14

Controlling flagellar rotation

• controlled by CheY protein
• -CheY results in counterclockwise rotation and runs
• -CheY-P results in clockwise rotation and tumbling

Question 15

Regulation of chemotaxis: Step 3

Answer 15

Adaptation

• feedback loop
• -Allows system to reset itself to continue to sense presence of a signal
• -involves modification of MCP’s

Question 16

Other taxes

Answer 16

Che proteins also play role in these:
Phototaxis: movement toward light
-light sensor replaces MCP's
Aerotaxis: movement toward oxygen
-redox protein monitors oxygen level

Question 17

Quorum sensing

Answer 17

Prokaryotes can respond to presence of other cells of same species.
Quorum sensing: mechanism by which bacteria assess their population density
-ensures sufficient cell density to have an effect (toxin production).

Question 18

Quorum sensing: Autoinducer

Answer 18

Each species of bacterium produces a specific AUTOINDUCER molecule

• diffuses freely across the cell envelope
• reaches high concentrations inside cell only if many cells are near
• binds to specific activator protein and triggers transcription of specific genes

Question 19

Quorum sensing: Classes of autoinducers

Answer 19

Several classes
-Acyl homoserine lactone first to be identified
First discovered regulating light production in bacteria including Aliivibrio fiscvheri
-Lux operon encodes bioluminescense

Question 20

Quorum sensing: Examples

Answer 20

Examples:
-P. aeruginosa switches from free living to growing as biofilm.
-virulence factors of staphylococcus aureus
Quorum sensing present in some microbial eukaryotes
Quorum sensing likely exists in archaea