L7: Regulation Of Gene Expression

Question 1

Prokaryotic Gene Expression

Answer 1

Highly regulated

Responds to surrounding nutrient levels & physical environment

At any given time, only genes encoding proteins required for survival under particular conditions are expressed

Question 2

Operons

Answer 2

Encode enzymes involved in particular metabolic pathway or proteins that interact -> multi-subunit protein

All genes transcribed as single mRNA- coordinately regulated

Question 3

Negatively regulated prokaryotic gene expression

Answer 3

Transcriptional repressors or gene repressor proteins. Bind specific sequences- operator regions

Ligands bind repressors and modulate their activity

Question 4

Positively regulated prokaryotic gene expression

Answer 4

Transcriptional activators or gene activator proteins. Bind specific sequences in promoter regions

Ligands bind activators and modulate their activity

Question 5

Lac operon operator regions

Answer 5

O1: main operator region, repressor binds at position +11 from transcriptional start site (in promoter region; where RNA polymerase binds)

O2 & O3: secondary operator regions, repressor bind at positions +412 and -82 from transcriptional start site, respectively

Question 6

Lac repressor

Answer 6

Single, tetrameric protein

Binds 2 operators simultaneously -> DNA between loops out

Strengthens interaction of Lac repressor with DNA. Greater levels of repression, RNA polymerase binding blocked

Question 7

Some bacterial gene regulatory proteins can act as repressors and activators

Answer 7

Dependent on location of binding sequence with respect to promoter of given gene. E.g. bacteriophage lambda repressor

For some genes, regulatory proteins act as activator - facilitates RNA polymerase binding

For other genes, regulatory protein acts as repressor- blocking RNA polymerase binding site

Question 8

Sigma factor

Answer 8

Required in transcription initiation in E.coli

Associate with RNA polymerase

Bind promoter sequence

Disengage from RNA polymerase after ~10 nucleotides of RNA are synthesised

Most common: sigma 70. Initiation of transcription of most genes (initiation factor)

Question 9

Alternative sigma factors

Answer 9

Required for transcription of some group of genes

Associate with RNA polymerase, replace sigma70

Recognise and bind specific promoter sequences

Most are structurally and functionally related to sigma 70

Question 10

E.coli sigma 54

Answer 10

Sequence different from those of sigma70 like factors

Sigma 54: contain RNA polymerase transcribing genes that are controlled by activators that bind to DNA 80-160 bp upstream (opp to direction of transcription) of transcription start site. Enhancer regions

Question 11

NtrC: a sigma 54 activator

Answer 11

Nitrogen regulatory protein C; dimeric protein

Stimulates transcription of gIn gene (codes for glutamine synthetase).

Initiation of transcription only after sigma 54 RNA polymerase is activated by NtrC

Question 12

NtrB activates NtrC

Answer 12

Protein kinase

Phosphorylates NtrC in response to low glutamine level -> NtrC binding at enhancer

Question 13

Two-component regulatory systems

Answer 13

Control bacterial responses to environment

Examples: NtrB & NtrC, PhoR and PhoB: regulate transcription of genes encoding proteins involved in phosphate metabolism

PhoR: transmembrane protein, periplasmic domain binds phosphate, cytosolic domain has protein kinase activity

PhoB: cytosolic protein

Question 14

PhoR and PhoB two component regulatory system

Answer 14

Low phosphate in environment -> low phosphate in periplasmic space

Phosphate dissociates from PhoR periplasmic domain

Conformational change in cytosolic domain -> activates protein kinase

Phosphate from ATP transferred to histidine residue on cytosolic domain

Some phosphate then transferred to PhoB

PhoB active -> transcription of several genes

Question 15

Two-component regulatory systems consist of 2 proteins..

Answer 15

Sensor protein: transmitter domain. E.g. PhoR & NtrB

Response regulator protein: receiver domain. E.g. PhoB and NtrC

Question 16

Sensor protein

Answer 16

E.g. PhoR and NtrB

Transmitter domain. E.g. PhoR and NtrB protein kinase domains. Shows specificity for receiver domain of response regulatory protein they phosphorylate

Regulated by second unique domain of protein e.g. PhoR periplasmic domain that sense environment

Question 17

Response regulatory protein

Answer 17

E.g. PhoB and NtrC

Receiver domain. e.g. PhoB and NtrC regions that are phosphorylated

Phosphorylation of receiver domain regulates activity of second functional domain. E.g. binding to regulatory region -> transcription

Question 18

Phase variation

Answer 18

Switching gene expression by DNA arrangement

Activation/inactivation of genes through DNA rearrangement. Occasional inversion of specific region of DNA in bacterial genome

Question 19

Salmonella

Answer 19

Example of phase variation

Genes encoding flagellin (protein subunits of flagellum)

Inversion changes orientation of promoter in inverted region

In one orientation: genes encoding H2 flagellin and repressor protein are transcribed. Repressor binds flagellin in H1 gene promoter and inhibits transcription
Other orientation: H2 & repressor genes are not transcribed. H1 gene transcribed

Outcome: bacterial cells able to escape immune response pf vertebrate host