Control of gene expression in prokaryotes: Operons - Week 7 Flashcards
what are the 2 different types of protein that are required
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
how can the expression of the inducible genes be regulated
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
what are the control points in gene regulation and why are they needed
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)
in bacteria how are genes for enzymes of metabolic pathway found and why it that important
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
why is the function of genes
genes code for operon enzymes; usually adjacent to each other; turn on one or they turn on all
what do operator, repressor and regulatory genes do
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.
what is an inducible operon
An inducible operon is one whose expression increases quantitatively in response to an enhancer, an inducer, or a positive regulator.
what does a lac operon regulate and when is it required
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.
when is lac operon turned on or off
The operon is turned “off” in the absence of lactose.
Operon is turned “on” in the absence of glucose and the presence of lactose.
what are the 3 genes that are in lac operon
The lac operon includes 3 structural genes which are lacZ, lacY and lacA. They are transcribed in unison.
how does lacl gene regulate the operon
The lacI gene regulates the operon by producing the lac repressor protein.
what does both the regulatory gene and lac operon contain and what happens there
Both the regulatory gene and the lac operon itself contain:
- promoters (Pl and Plac) at which RNA polymerase binds
- terminators at which transcription halts.
what happens when plac (a promoter) overlaps with the operator site
When Plac overlaps with the operator site then the active form of the repressor protein binds.
what is the function of lacZ
lacZ + Beta-Galactosidase breaks the glycosidic bond between glucose and galactose.
what is the function of lacY
lacY is a lactose Permease Enzyme that spans the membrane and brings lactose into the cell.
what is the function of lacA
lacA is a Thiogalactoside transacetylase, its function is unknown but it may play a possible role in the detoxification of some lactose metabolites.
where are lacZ, lacY and lacA transcribed
The genes lacZ, lacY and lacA are transcribed from a single promoter that produces a single mRNA from which the three proteins are translated.
how is the transcription of lac operon downregulated
Transcription of the lac operon is down-regulated through the binding of the lac repressor to the operator. The lacI gene is always expressed.
what happens in the absence of lactose
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.
what happens when lactose is present
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.
what type of protein is the lac repressor protein
The lac repressor is an allosteric protein capable of reversible conversion between two alternative forms.
what does lac operon respond to in positive and negative control
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.
what substances mediate positive control of lac operon
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.
how is the level of cAMP related to the level of glucose
The level of cAMP is inversely related to the level of glucose.
Because as glucose levels drop, cAMP levels rise and vice versa.
how is transcription upregulated in lac operon
Transcription of the lac operon is up-regulated through the binding of the cAMP-CAP complex to the promoter.
what type of protein is CAP and when is it active and inactive
CAP is an allosteric protein that is inactive in the free form but is activated by binding to cAMP.
how is transcription stimulated
The CAP-cAMP complex binds the promoter of inducible operons, increasing the affinity of the promoter for RNA polymerase and thus stimulating transcription.
how does RNA polymerase bind to the promoter and what happens after it binds
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.
