Lecture 9 Transcriptional Regulation of Gene Expression

-to-% of our genome codes for proteins

1 to 2

in multicellular organisms, gene expression is under extremely

elaborate control

even bacterium express genes __

selectively to make enzymes needed to digest food only when available

through embryonic development, a fertilized egg cell gives rise to many

cell types that differ in structure and function

differences between a mammalian neuron and a hepatocyte for example are so

extreme that it is difficult to imagine that the two cells contain the same DNA

however, nearly all the cells of a multicellular organism contain the same

genome

cell differentiation is instead achieved by

changes in gene expression

controlling gene expression in cells slide

__ is responsible for the diversity in cell type

differential gene expression

eukaryotic gene expression can be regulated at

multiple levels

only _ genes out of the 25,000 genes expressed in a typical human cell

5,000-15,000

_ is main step of regulation for most genes

transcriptional regulation

some genes expressed in both liver and neural cells while others are

only expressed in one

the regulation of gene expression for multiple levels gives most control over

genome at any given time

regulation of gene expression from multiple levels slide

transcription is controlled by

proteins binding to regulatory DNA sequences

a transcription factor does what

binds to promoter regions and activates or represses

in addition to promoter, nearly all genes, whether bacterial or eukaryotic, have regulatory

DNA sequences that are used to switch the gene on or off

regulatory DNA sequences are recognized by

proteins called transcription regulators to control transcription

where does a transcription regulator bind on DNA

to the major groove of a DNA helix

typical protein-DNA interface consists of

10-20 contacts, each involving a different amino acid each contributing to strength of protein-DNA interaction

transcriptional regulators interact with DNA in a

specific place and way

these features will vary depending on

nucleotide sequence, thus different proteins will recognize different nucleotide sequences

proteins forms —,—,— with edges of bases

hydrogen bonds, ionic bonds, and hydrophobic interactions

around how many contacts typically formed at protein-DNA interface combine for interaction both highly specific and strong

20

_ interactions are among tightest and most specific molecular interactions

protein-DNA

transcription regulator binding spot slide

many proteins responsible for gene regulation recognize DNA through

several structural motifs

structural motifs typically scene in proteins responsible for gene regulation

homeodomain, zinc finger, leucine zipper

motifs of transcriptional regulators slide

a cluster of _ can be transcribed from a single promoter

bacterial genes

transcription switches allow cells to respond to

changes in environment

clusters of genes transcribed as a single mRNA molecule are common as bacteria called

an operon

expression of tryptophan operon is controlled by

a regulatory DNA sequence called the operator situated within the promoter

in prokaryotic transcriptional events that express an operon expresses all the parts that are needed for

a function

operator =

piece of DNA sequence a transcriptional regulator can bind

bacterial genes being transcribed from a single promoter slide

genes can be _ with repressor proteins

switched on and off

when tryptophan is low

RNA polymerase binds to promoter and transcribes five genes of tryptophan operon

when tryptophan is high

repressor protein becomes active and binds to operator where it blocks the binding of RNA pol to promoter

whenever tryptophan drops

repressor releases its tryptophan and falls off DNA, allowing polymerase to transcribe operon

operons put together

multiple genes for a particular process

microbes _ own tryptophan when it gets low

produce

when tryptophan levels are high tryptophan does what

binds to represor protein to activate, repressor protein then becomes activated and binds to operator and blocks RNA pol from binding

tryptophan slide

gene expression can also be controlled by

activator proteins

what does an activator protein do

binds to regulatory sequence on DNA and interacts with RAN pol to help initiate transcription efficiently

without activator promoter _

fails to initiate transcription efficiently

bacterial catabolite activator protein must do what before it can bind to DNA

bind cyclic AMP (cAMP)

lac operon is controlled by _ signals:

2 signals: glucose nd lactose

+glucose +lactose

nothing binds, operon is off

+glucose -lactose

repressor is bound, operon off

-glucose, -lactose

cyclic amp and CAP bound, repressor bound, operon off

-glucose, +lactose

CAP bound w/ cyclic AMP, RNA pol can bind, operon on

Glucose and lactose concentrations control

initiation of transcription of Lac operon through effects on Lac repressor protein and CAP.

Lactose is absent,

Lac repressor binds Lac operator and shuts off expression of operon.

Glucose is absent,

cyclic AMP (red triangle) is produced by cell and CAP binds to DNA. LacZ, first gene of operon, encodes enzyme β-galactosidase: breaks down lactose to galactose and glucose

Addition of lactose increases

intracellular concentration of related compound, allolactose. Allolactose binds to the Lac repressor, causing it to undergo a conformational change that releases its grip on operator DNA (not shown).

lac operon slide

in eukaryotes gene activation occurs how

at a distance

an activator protein bound to DNA does what

attracts RNA polymerase and general transcription factors to the promoter

looping of the DNA permits what

contact between the activator protein bound to the enhancer and the transcription complex bound to the promoter

what serves as the go between

large protein complex called mediator

eukaryotic gene activation slide

eukaryotic gene activator proteins can direct _ alterations in chromatin structure

local

activator proteins can recruit —- and —- to promoter region of DNA

histone-modifying enzymes, chromatin-remodeling complexes

histone-modifying enzymes and chromatin-remodeling complexes makes DNA packaged in —- more —

make DNA packaged in chromatin more accessible to other proteins

the covalent histone modifications can serve as

binding sites for proteins that stimulate transcription initiation

eukaryotic gene activator slide 1

many transcription activators attract histone acetylases which do what

attach an acetyl group on tail of histone proteins

histone acetylases alter

chromatin struccture allowing greater accesibility to DNA

many repressors attract

histone deacetylases enzymes that remove acetyl groups from histone tails, thereby reversing the positive effects that acetylation has on transcription initiation

histone acetylases slide