Genetics final Lecture 5

Lactose

Sugar is a source for bacteria when broken down into glucose. To convert lactose into glucose, the enzymes permease and B-galactosidase are needed.

Permerase

Transport lactose in cell. Allows lactose to enter the cell

B-galactosidase (B-gal)

Splits lactose into glucose and galactose

Lactose absense

Basal expression of permease and B-galactosidas. Repressor protein is bound to the operator.

Lactose presence

High expression of permease and B-galactosidase. Lactose is an inducer of the enzymes.

-Allolactose binds to the repressor, changing its shape and preventing it from binding to the operator.

a. RNA polymerase binds to promotor and initiates transcription of polycistronic mRNA

Inducer

Responsible for inducing the expression of a gene

Induction

Stimulation of synthesis of a specific protein

Lac-

Mutant strain of bacteria that cannot utilize lactose as a food source

Operon theory

One signal can simultaneously regulate expression of several clustered genes

Lac operon

Lac genes are transcribed together as a single mRNA (polycistronic) from a single promoter

Lactose operon structural genes

LacZ, LacY, ans LacA. All three genes are under the control of a single promoter

Operator

Between structural gene and promoter. Is critical in regulating expression of lac operon

Operon

cluster of genes regulated by a single promoter (regulatory region)

LacI

Encodes the lac repressor protein, a crucial regulator for the lac operon in bacteria like E. coli.

Repressor gene that regulates the lac operon.

Encodes a protein that binds to the operator region and prevents RNA polymerase from transcribing. Leads to low expression

LacZ

codes for b-galactosidase

LacY

Codes for permease

LacA

Codes for transacetylase

Constitutive expression

Expressed all the time. Does not depend on the presence of lactose

Repressor

removed when inducer is present

A mutant that does not produce lacI shows

Expression of lac operon in the presence or absence of lactose. Constitutive expression, it is no longer regulated

X-gal

Turns blue if theres B-galactosidase

Constitutive mutants (lacI-)

Express the enzymes in the absence and presence of inducer. Constitutive expression.

a. mutant repressor cannot bind to operator allowing RNA polymerase to bind

When can constitutive expression

• mutation in LacI-

• Mutation in operator

lacO^c mutation

Mutant operator that cannot bind the repressor. constitutive expression

lacI^s

superrepressor

Inducer cannot bind to the repressor.

Basal expression in the presence or absence of lactose

Transacting element

Proteins that can bind to any number of DNA site on any DNA molecule in the cell. Can diffuse through the cytoplasm once formed

• lacI

Proteins act in trans.

Can regulate multiple operons

Cis-acting element

Regulatory DNA sequence

• promoter, operator

It can influence the expression of adjacent genes on the same DNA molecules.

Mutation only impact genes its physically connected to.

DNA sequences act in cis