Control of gene expression in prokaryotes: Operons - Week 7

Question 1

what are the 2 different types of protein that are required

Answer 1

Some proteins are required continuously – the genes encoding them are expressed constitutively

Many other proteins are required for specific purposes – the genes are inducible under particular conditions

Question 2

how can the expression of the inducible genes be regulated

Answer 2

Expression of inducible genes can be either positively or negatively regulated:
- Positive regulation (induction) is often used in the catabolism of substrates for energy production – the presence of the substrate stimulates the expression of genes in the metabolic pathway
- Negative regulation (repression) is used in anabolic pathways, whereby the absence of the substrate represses the expression of the genes in the metabolic pathway

Question 3

what are the control points in gene regulation and why are they needed

Answer 3

There are several potential control points in the gene expression pathway, which are:
>Transcriptional regulation
>Post-transcriptional regulation
- includes co-transcriptional control –RNA splicing
- mRNA stability and transport
>Translation
>Post-translational regulation
- protein stability
- activation or inactivation (e.g phosphorylation,
ADP-rybosylations etcs)

Question 4

in bacteria how are genes for enzymes of metabolic pathway found and why it that important

Answer 4

in bacteria, genes for enzymes of metabolic pathways are usually clustered in a functional complex under coordinated control
Grouping related genes under a common control mechanism allow bacteria to rapidly adapt to changes in the environment

Question 5

why is the function of genes

Answer 5

genes code for operon enzymes; usually adjacent to each other; turn on one or they turn on all

Question 6

what do operator, repressor and regulatory genes do

Answer 6

An operator is the repressor protein binding site.
Repressor is a gene-regulatory protein which binds to the operator.
A regulatory gene encodes repressor protein.

Question 7

what is an inducible operon

Answer 7

An inducible operon is one whose expression increases quantitatively in response to an enhancer, an inducer, or a positive regulator.

Question 8

what does a lac operon regulate and when is it required

Answer 8

lac operon regulates the production of the enzymes needed to degrade lactose in bacterial cells.
It is required when bacteria have no available glucose but have available lactose as an energy source.

Question 9

when is lac operon turned on or off

Answer 9

The operon is turned “off” in the absence of lactose.
Operon is turned “on” in the absence of glucose and the presence of lactose.

Question 10

what are the 3 genes that are in lac operon

Answer 10

The lac operon includes 3 structural genes which are lacZ, lacY and lacA. They are transcribed in unison.

Question 11

how does lacl gene regulate the operon

Answer 11

The lacI gene regulates the operon by producing the lac repressor protein.

Question 12

what does both the regulatory gene and lac operon contain and what happens there

Answer 12

Both the regulatory gene and the lac operon itself contain:
- promoters (Pl and Plac) at which RNA polymerase binds
- terminators at which transcription halts.

Question 13

what happens when plac (a promoter) overlaps with the operator site

Answer 13

When Plac overlaps with the operator site then the active form of the repressor protein binds.

Question 14

what is the function of lacZ

Answer 14

lacZ + Beta-Galactosidase breaks the glycosidic bond between glucose and galactose.

Question 15

what is the function of lacY

Answer 15

lacY is a lactose Permease Enzyme that spans the membrane and brings lactose into the cell.

Question 16

what is the function of lacA

Answer 16

lacA is a Thiogalactoside transacetylase, its function is unknown but it may play a possible role in the detoxification of some lactose metabolites.

Question 17

where are lacZ, lacY and lacA transcribed

Answer 17

The genes lacZ, lacY and lacA are transcribed from a single promoter that produces a single mRNA from which the three proteins are translated.

Question 18

how is the transcription of lac operon downregulated

Answer 18

Transcription of the lac operon is down-regulated through the binding of the lac repressor to the operator. The lacI gene is always expressed.

Question 19

what happens in the absence of lactose

Answer 19

In the absence of lactose, the repressor remains bound to the operator and prevents access of the RNA polymerase to the promoter. Transcription is blocked and the operon is repressed thus RNA polymerase is prevented from moving down the lac operon and transcribing its genes.

Question 20

what happens when lactose is present

Answer 20

when lactose is present, the repressor doesn’t bind to the operator and thus the operon is derepressed.
in the presence of lactose, the repressor is converted to its inactive form thus the RNA polymerase can move past the operator and transcribe lacZ, lacY and lacA genes into a single mRNA.

Question 21

what type of protein is the lac repressor protein

Answer 21

The lac repressor is an allosteric protein capable of reversible conversion between two alternative forms.

Question 22

what does lac operon respond to in positive and negative control

Answer 22

With negative control, lac operon would respond only to the presence of lactose.

whereas in Positive control lac operon will select glucose metabolism if both glucose and lactose are present.

Question 23

what substances mediate positive control of lac operon

Answer 23

The positive control is mediated by a substance that senses glucose levels inside the cell, which are:
- cyclic AMP (cAMP)
- positive regulator protein called the catabolite activator protein (CAP), which is active only in the presence of cAMP.

Question 24

how is the level of cAMP related to the level of glucose

Answer 24

The level of cAMP is inversely related to the level of glucose.
Because as glucose levels drop, cAMP levels rise and vice versa.

Question 25

how is transcription upregulated in lac operon

Answer 25

Transcription of the lac operon is up-regulated through the binding of the cAMP-CAP complex to the promoter.

Question 26

what type of protein is CAP and when is it active and inactive

Answer 26

CAP is an allosteric protein that is inactive in the free form but is activated by binding to cAMP.

Question 27

how is transcription stimulated

Answer 27

The CAP-cAMP complex binds the promoter of inducible operons, increasing the affinity of the promoter for RNA polymerase and thus stimulating transcription.

Question 28

how does RNA polymerase bind to the promoter and what happens after it binds

Answer 28

The CAP-cAMP complex makes the promoter more readily bound by RNA polymerase because as CAP binds to the promoter it causes the promoter to bend 90º which allows the attachment of the RNA polymerase.
When RNA polymerase binds to the promoter it transcribes the operon.