Operons

gene regulation

involves expression changes that may alter amino acid sequences, impacting protein structure and function

replication level

transcriptional level

post-transcriptional level

translational level

Gene Regulation Levels

replication level (gene regulation)

any error in copying the DNA may result in an altered expression

transcriptional level (gene regulation)

any error in the polymerization may again lead to a change in expression of the gene

post-transcriptional level (gene regulation)

RNA splicing

translational level (gene regulation)

any error in the attachment of mRNA to the tRNA molecules, there may arise some changes

sigma factors

are prokaryotic proteins that bind to specific promoters

transcription

the promoter recruits RNA polymerase to a gene which initiates:

sigma 70 (prokaryotic transcription regulation)

promotes transcription of genes involved in cell growth and division

sigma 32 (prokaryotic transcription regulation in prokaryotes)

promotes transcription of genes that respond to heat stress

at the initiation of transcription

gene regulation in prokaryotes is most extensively observed where?

in the expression of the RNA polymerase at the promoter site

transcription regulation in prokaryotes usually takes place where?

positive regulation by activators

negative regulation by repressors

these accessory proteins can regulate the promoter site in two ways:

operon

a group of functionally related genes located together in a genome, typically found in prokaryotes

operon

consists of a promoter, an operator, and one or more structural genes that encode protein

operator

additional regulatory region

operon

includes regulatory elements of the transcript

Lac operon

three genes lacZ, lacY and lacA required for metabolizing lactose through the activity of β-galactosidase

Z gene

codes for β-galactosidase which catalyzes the hydrolysis of lactose into glucose and galactose

β-galactosidase, permease, and transacetylase

What three proteins are needed for the metabolism of lactose

y gene

codes for permease which regulates the lactose permeability in the cell

A gene

code for transacetylase which assists the enzyme beta-galactosidase

lac operon

contains genes involved in metabolism

only when lactose is present and glucose is absent

when are the genes in lac operon expressed

catabolite activator protein and lac repressor

The operon is turned on and off in response to the glucose and lactose levels:

lac repressor

blocks the transcription of the operon. In the presence of lactose, it stops acting as a repressor

catabolite activator protein (CAP)

activates the transcription of the operon, only when glucose levels are low

single mRNA molecule

In lac operon, all of the genes are transcribed as a:

proteins involved in lactose metabolism

each gene in lac operons are translated into what?

lactose

acts as an inducer, inducer will bind to the repressor protein (negatively regulated)

lacl

codes for a repressor protein

constant or constitutive expression

repressor protein

binds to the lac operon regulatory region called operator (“O”)

lac repressor protein

also binds allolactose (isomer of lactose)

inactive repressor (lac operon is transcribed)

Lac repressor protein binding to allolactose leads to what?

CRP (cyclic AMP receptor protein)

activator protein

CRP

receptor protein that forms complex with cyclic AMP (cAMP) and binds to a specific site near lac promoter

CRP

promote transcription of lac operon

cAMP

is made when glucose levels drop

cAMP

this regulator mechanism depends on the availability of glucose as a carbon source for the bacterium

increase lac operon expression

lactose increase + glucose decrease equals what?

glucose

is preferred of lactose as the fuel source

inducible operon

typically, off

transcription is activated (induced by an inducer molecule) (ex. lactose)

the absence of glucose

expression from lac operon is induced in the presence of lactose and what?

trp operon

enzymes involved in the synthesis of the amino acid tryptophan

trp operon

is a well-known example of a repressible operon found in E. coli and other bacteria

trp operon

it controls the production of tryptophan, an essential amino acid

repressible operon

type of operon that is always on, allowing constant gene transcription and protein production

the repressor protein is inactive

RNA polymerase can bind to the promoter and initiate transcription of the operon

in the absence of tryptophan, what happens to the repressor protein and transcription of operon?

act as co-repressors and bind to the repressor protein, activating it

when tryptophan levels are high, tryptophan molecules do what?

trp repressor is active when bound to tryptophan

activated trp repressor inhibits transcription of the trp operon

Repressible operon steps

operons

clusters of functionally related genes controlled by a single promoter