GENETIC REGULATION IN EULARYOTES

LEVELS

1.Level of transcription Trans-acting molecules Cis-acting elements 2. Posttranscriptional level Alternative mRNA splicing Control of mRNA stability Control of mRNA editing 3. DNA level Access to DNA Amount of DNA Arrangment of DNA Mobile DNA elements

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I.LEVEL OF TRANSCRIPTION TRANS ACTING MOLECULES

Specific transcription factors(STF)• Two binding domains: 1. The DNA-binding domain• contains specific structural motifs, such as zinc fingers

that bind sequences in DNA. 2. The transcription-activation domain recruits the general

transcription factors ([GTFs] • These facilitate formation of the transcription initiation

complex (RNA polymerase II plus the GTFs) at the promoter, and, thus, activate transcription

CIS ACTING ELEMENTS

• The area on DNA where the DNA binding domain of the specific transcription factors binds

• Eg, Hormone-Response Elements (HREs) are cis-acting DNA sequences that bind trans-acting protein factors and regulate gene expression in response to hormonal signals.

INTRACELLULAR RECEPTOR

• Steroid hormone (glucocorticoids, mineralocorticoids, androgens, and estrogens), vitamin D, retinoic acid, and thyroid hormone receptors

• DNA-binding domain • Activation domain• Ligand-binding domain.

STEPS• Cortisol bind to soluble, intracellular receptors at the ligand-

binding domain.• Binding causes a conformational change in the receptor

that activates it. • The receptor–ligand complex enters the nucleus, dimerizes• Binds via a zinc finger motif to nuclear DNA at a cis-acting

regulatory element, the glucocorticoid-response element (GRE).

• Binding allows recruitement of coactivators to the activation domain and results in increased expression of cortisol-responsive genes

Intracellular receptor

CELL SURFACE RECEPTOR• Insulin, epinephrine, and glucagon.

STEPS:• Hormone binds its G protein–coupled plasma membrane

receptor • Increased intracellular cAMP • Protein kinase A–mediated phosphorylation• A trans-acting factor (cAMP response element–binding [CREB]

protein) is phosphorylated and activated. • Active CREB protein binds via a leucine zipper motif to a cis-

acting regulatory element, the cAMP response element (CRE)• Transcription of target genes with CREs in their promoters.

Cell surface receptor

II. Regulation by processing of messenger RNA

1. Splice site choice2. mRNA editing3. mRNA stability

1.Splice site choice

Tissue-specific protein isoforms can be made from the same pre-mRNA through differential use of alternative splice sites

eg.Tropomysin

Alternative splicing

mRNA editing

• A base in the mRNA is altered• Example:• Apo B mRNA is made in the liver and the small intestine.• In the intestine only, the C residue in the CAA codon for

glutamine is deaminated to U, changing the sense codon to a nonsense or stop codon

• This results in a shorter protein (apo B-48) being made in the intestine (and incorporated into chylomicrons)

• The one made in the liver (apo B-100, full-length, incorporated into VLDL).

mRNA editing

mRNA stability

• How long an mRNA remains in the cytosol before it is degraded influences how much protein product can be produced from it.

• Eg Iron metabolism

mRNA stability

TRANSLATION OF mRNA

• phosphorylation by kinases of the eukaryotic translation initiation factor, eIF-2

• This inhibits its function and so inhibits translation at the initiation step

III.Regulation through modifications to DNA

• Access to DNA• Amount of DNA• Arrangment of DNA• Mobile DNA elements

1.ACCESS TO DNA

• DNA is found complexed with histone and nonhistone proteins to form chromatin

ACTIVE DNA (euchromatin):• Acetylation (HAT)• Phosphorylation• Hypomethylation• Nucleosome repositioned(chromatin

remodeling)

2.AMOUNT OF DNA

• A change up or down in the number of copies of a gene can affect the amount of gene product produced.

• An increase in copy number (gene amplification) has contributed to increased genomic complexity

• Seen in diseases and with MTX

3.DNA REARRANGEMENTSFig. DNA rearrangements in the generation of immunoglobulins.

V= variable; D = diversity; J = joining

4.MOBILE DNA• Transposons (Tns) are mobile segments of DNA that move in an

essentially random manner from one site to another on the same or a different chromosome.

• Movement is mediated by transposase, an enzyme encoded by the Tn itself.

• Movement can be : Direct, in which transposase cuts out and then inserts the Tn at a

new site Replicative, in which the Tn is copied and the copy inserted

elsewhere while the original remains in place. • Replicative transposition frequently involves an RNA

intermediate, in which case the Tn is called a retrotransposon . • Can alter gene expression and even to cause disease• Eg, hemophilia, duchene dystrophy

DIRECT MOVEMENT

REPLICATIVE MOVEMENT