genetics lec 18 regulation gene expression in prok

constitutive genes

• always active

• housekeeping

• products essential for growth

regulated genes

• products only at specific times

• controlled activity

• expressed in specific context

What are the mechanisms regulated genes use?

• repressor & activator proteins

• attenuation

• antisense RNA

• riboswitch

Define operon

a cluster of genes coordinately regulated by one promotor

Lactose expresses how many genes?

3; beta-gal, permease, transacetylase

Function of beta-gal

metabolizes lactose

Function of permease

bring lactose into cell

Define constitutive mutant

proteins are produced all the time

LacI

• controls inducibility

• regulatory gene

The I gene encodes

makes protein that negatively controls beta-gal gene

What is Negative control

repressor blocks txn

presence of inducer overcomes block

Enhancer

a DNA sequence that “enhances” gene expression by binding transcription factors; far away

Promotor

Where RNAP binds to start txn

activator

a protein that increases the transcription of a gene; helps RNAP bind to promotor

Inducer

a molecule that turns on gene expression by binding to a repressor protein

Ex: lactose

Repressor protein (I) has two sites

1. DNA binding site

2. Allolactose binding site

I^s

can’t bind allolactose; never txn

operator

• Next to the promoter within an operon.

• A binding site for repressor proteins.

O^c

constitutive operator; always txn

Nonsense mutations in operon cause…

• causes ribosome to stop translation early

• downstream products not made

missense mutation

not affect downstream products

creates non-functional protein

describe the CAP:cAMP system

CAP is inactive on its own, when cAMP binds to CAP, txn occurs b/c RNAP can bind to promotor.

what happens if there is glucose in the system

cAMP is inhibited by glucose → no CAP:cAMP complex → no txn since no binding of RNAP

Tryptophan Operon has 2 types of control, name them

1. Negative control

2. Attenuation

Describe neg control of trp

repressor binds to operator when trp is present → no txn

What is Attenuation

regulation method that causes premature termination of transcription (or sometimes translation) based on the cell's needs, by forming hairpins that either stop or allow the ribosome to continue

Where does attenuation occur

5’ UTR of mRNA

What does the 5’ UTR contain

• 4 regions

• open reading frame with trp codons for ribosome to bind

why does the ribosome stay in place at region 1

it is waiting for a charged tRNA with trp

when trp levels are low…

2+3 form stem loop, continues txn

when trp levels are high…

ribosome continues moving down → 3+4 makes stem loop that causes RNAP to stop

How is gene expression regulated by RNA

1. Antisense RNA

2. Riboswitches

What is antisense RNA

it inhibits txn by binding complementary sequence in mRNA

What is a riboswitch

a regulatory sequence in mRNA where inducers or repressors can bind to

forming a 2ndary structure with stem loops prevents translation