gene expression

Question 1

regulation of gene expression in bacteria

Answer 1

• a bacterium adapts depending on the environment
• genes that are not required are not expressed unless environmental conditions change (regulation is very tight)

Question 2

levels of gene regulation

Answer 2

• alteration of structure
• transcription
• mRNA processing
• RNA stability
• Translation
  -post-translational modification

Question 3

constitutively expressed genes

Answer 3

genes that specify products that are essential components of living cells
- need all the time and are always turned on

Question 4

inducible and repressible genes

Answer 4

• needed for cell growth or only under certain environmental conditions
• regulatory mechanisms allow the synthesis of these gene products only when they are needed

Question 5

ligand

Answer 5

binds to a repressor activator and tells it these are the conditions under which you should bind to the DNA

Question 6

Negative control: operon types

Answer 6

Repressible: default mode is on
inducible: default mode is off

Question 7

Negative control: operon types

Answer 7

Repressible: default mode is on
inducible: default mode is off

Question 8

positive regulation in bacteria

Answer 8

• involves an activator which when bound allows transcription of mRNA

Question 9

positive inducible genes

Answer 9

• usually off (activator not bound)
• in absence of ligand binding to activator, gene is turned off
• if ligand binds to activator, activator will bind to promotor and transcription will happen

Question 10

positive repressible genes

Answer 10

• usually off (activator bound)
• when ligand binds to activator it leaves the promoter and transcription no longer happens

Question 11

negative regulation in bacteria

Answer 11

• repressor is involved which when bound prevents transcription of mRNA

Question 12

negative repressible genes

Answer 12

• usually on (repressor not bound)
• when ligand binds to repressor, repressor binds and strops transcription

Question 13

negative inducible

Answer 13

• usually off (repressor bound)
• when ligand binds to repressor it leaves and transcription can happen

Question 14

Positive control: CAMP and CRP

Answer 14

• low glucose causes buildup of cyclic AMP (cAMP)
• camp binds to regulatory protein CRP
• this activates the protein which binds to promoter of the operon
• allows RNA polymerase to bind and begin transcription (turns on)
• as glucose levels rise cAMP levels lower, decreasing rate of transcription

Question 15

induction of genes for lactose utilization

Answer 15

• for utilization of lactose gene expression is induced when lactose is present and glucose is absent
• alters the rate of lactose-metabolizing enzyme synthesis
• enzymes involved in catabolic pathway are often inducible (off)

Question 16

repression of genes for tryptophan biosynthesis

Answer 16

• genes are expressed in the absence of tryptophan and turned off when tryptophan is available
• enzymes involved in anabolic pathways are often repressible (on)

Question 17

operon

Answer 17

a series of related genes that are expressed coordinately through the use of a single promoter and operator
- most structural genes with related functions in bacteria are organized into operons for efficiency of gene expression

Question 18

negative inducible operons

Answer 18

• usually off
• the regulator protein is active and bound to the operator and prevents transcription of the structural genes
• then the inducer is present it binds to the regulator making it unable to bind to the operator and transcription takes place

Question 19

inducer

Answer 19

ligand that helps to express an operon
- may bind to repressor to take it off or bind to activator to put it on

Question 20

negative repressible operons

Answer 20

• usually on
• regulator protein is an inactive repressor, unable to bind to operator, transcription takes place
• levels of corepressor build up which bind to regulator protein making it active and able to bind to operator, preventing transcription

Question 21

positive control of gene expression

Answer 21

the regulatory protein “activator” binds to DNA and interacts with RNA polymerase to assist the efficiency or transcription of structural genes

Question 22

The lac operon of E.coli

Answer 22

• makes lactase to break down lactose
• genes are transcribed only when lactose is present and glucose is absent
• consists of regulatory repressor “I” gene (contains its own promoter), a second promoter “P”, a regulatory operator “O”, and 3 structural genes (Z,Y,A)

Question 23

operator

Answer 23

• a piece of sequence that sits after the promoter and binds to repressor

Question 24

lac operon system

Answer 24

• lactose premease (Y gene): lactose transporter (in membrane)
• B-galactosidase (Z gene): main purpose is to turn lactose into glucose and galactose, low background level converts lactose (allolactose - lac operon inducer)
• lac I region: encodes lac repressor, diffusible tetramer that represses in the absence of all lactose
• lac O region: the lac operator, regulatory DNA sequence that works in cis to control lac operon, binding site

Question 25

lac operator

Answer 25

- overlaps the initiation start site - when bound by lac repressor RNA poly can't access promoter

Question 26

when is the lac operon ON or OFF

Answer 26

ON: in the presence of allolactose - lac repressor binds inducer and is inactive - induction results in transcription of lac Z, Y, and A genes OFF: in the absence of the inducer allolactose - the lac repressor binds to the lac operator and prevents RNA poly from transcribing Z, Y and A

Question 27

what happens when there is a mutation in the lac operator

Answer 27

- genes are always on

Question 28

constitutive lac mutants

Answer 28

- caused by mutations in the I gene (I-) or the operator (Oc) - Oc mutations act only in cis - wild type Z+, Y+ and A+ are dominant (from functional enzymes) and I+ is dominant

Question 29

lacl superrepressors

Answer 29

- some lac mutants were never inactivated by allolactose - in these mutants the allolactose binding site in the lac repressor gene is altered - lacl repressor is always bound to the operator, preventing transcription of genes - in a diploid, Lacls is dominant over lacl+

Question 30

Is the lac operon always induced in the presence of allolactose

Answer 30

- no, not if glucose is also present - glucose prevents lac operon induction- negative regulation - aka catabolite repression

Question 31

how does glucose regulate the lac operon

Answer 31

- indirectly through cAMP levels - when glucose is high, cAMP is low - when cAMP is high (low glucose), it binds to CAP which permits binding to a special region in the lac promoter - required for lac operon transcription

Question 32

positive control of the lac operon (low glucose)

Answer 32

1) cAMP/CAP binding 2) RNA poly binding - cAMP/CAP orients RNA poly and associated sigma factor to start transcription