ch 18: regulation of gene expression

differential gene expression

expression of diff genes, allowing cells to carry out their specific function

how can bacteria cells regulate enzymes?

feedback inhibition or gene regulation

how does bacteria regulate gene expression?

through operons

feedback inhibition

controls activity of existing enzymes

gene regulation

controls enzyme production

operon

entire stretch of DNA that includes the operator, promoter, and genes that they control

promoter

where RNA polymerase binds

operator

an “on-off” switch that controls transcription

genes

encodes functionally related proteins

repressor

can bind to the operator and turn transcription off

corepressor

cooperates/ w a repressor protein to switch an operon off

repressible operon

usually ON; binding of a repressor to the operator shuts OFF transcription

repressible operon example?

trp operon

inducible operon

usually off; an inducer inactivates the repressor and turns ON transcription

inducible operon example?

lac operon

tryptophan

when absent → operon ON

when present → operon OFF

acts as a corepressor by activating the repressor

lactose

when absent → operon OFF

when present → operon ON

allolactose acts as an inducer by inactivating the repressor

which stages can gene expression be regulated at?

• chromatin modification

• transcription

• RNA processing and degradation

• translation

• protein processing and degradation

positive regulation

an activator increases transcription by helping RNA polymerase bind to DNA

cAMP receptor protein (CRP)

an activator of the lac operon

cAMP (cyclic AMP)

a small signaling molecule that binds to and activates CRP

how does glucose availability control cAMP levels?

low gluose → high cAMP → active CRP → high transcription

high glucose → low cAMP → inactive CRP → low transcripton

what affects DNA accessibility for transcription?

chromatin structure

are genes with highly packed heterochromatin expressed?

no

histone acetylation

addition of acetyl groups to histone tails

how does acetylation increase transcription?

acetylation → looser chromatin → DNA more accessible → more transcription

should DNA be compact or loose to be accessible for transcription?

loose

DNA methylation

the addition of methyl groups to DNA; reduces transcription; can contribute to long-term gene silencing during cell differentiation

do chromatin modifications change DNA sequence?

they can affect gene expression without changing DNA sequence

epigenetic inheritance

modifications passed to future generations of cells

control elements

noncoding DNA sequences that bind transcription factors

proximal control elements

near the promoter

distal control elements

may be far from the gene

enhancer

group of distal control elements

what is the difference between general transcription factors vs specific transcription factors?

general factors - help transcription to happen vs specific factors - help determine which genes are highly expressed, when, and where

general transcription factors

required for transcription initiation; assemble w/ RNA polymerase II at the promoter

specific transcription factors

bind specific control elements; regulate transcription of particular genes; activators can bind enhancers

activator

a transcription factor that binds an enhancer and stimulates the transcription

how does an activator increase transcription?

activator binds enhancer → DNA bends → transcription machinery assembles → transcription increases

different cell types contain different activators →

different genes are expressed

can gene expression still be regulated after transcription?

yes - RNA processing, mRNA degradation, translation, protein processing and degradation

what happens if the primary RNA transcript is spliced in diff ways?

diff combos of exons → diff mRNAs → diff proteins

regulatory proteins

can block or promote translation initiation

what controls whether an mRNA is translated?

cells

what affects how much protein can be produced?

the life span of an mRNA (longer-lived → more opportunities for translation)

what helps influence mRNA stability?

the poly-A tail

mRNA stability

stable mRNA → more translation → more protein

rapid mRNA degradation → less translation → less protein

ubiquitin

tags proteins for degradation

proteasomes

recognize and degrade tagged proteins

how does a protein become degraded?

protein → ubiquitin tag → proteasome → degradation