Gene regulation biology 1 lecture 13

Differential expression of genetic code in prokaryotes and eukaryotes Regulation at the transcription level How DNA is read - regulatory proteins Motifs Control by polymerase blocking in prokaryotes Control ‘at a distance’ in eukaryotes Post-transcriptional control

Gene expression is controlled by regulating transcription Regulation of promoter access Transcriptional control in prokaryotes is more straight forward Bacteria configure expression of genes to fit environment; generally reversible –RNA polymerase must bind to promoter –Protein regulators bind to promoter to either Inhibit binding of RNA polymerase Facilitate binding of RNA polymerase

Regulatory proteins read DNA without unwinding it Regulatory proteins bind to major groove of DNA helix, reading base pairs outside edge Several motifs are seen that bind to the DNA molecule –Helix-turn-helix Homeodomain –Zinc-finger –Leucine zipper

Prokaryotes limit transcription by blocking the polymerase Repressors are off-switches –e.g., tryptophan producing [trp] genes in E.coli: block of trp operon at promoter prevents transcription of 5 genes related to enzymes needed to make tryptophan –Presence of tryptophan shuts down transcription

Activators are on-switches –e.g, catabolite activator protein (cap) of E.coli. –Decrease of glucose leads to increase in cAMP –cAMP binds to CAP, protein changes shape –CAP's helix-turn-helix motif binds to DNA near several promoters –Promoters activates, genes transcribed

Switches combine to form complex control systems e.g., the lac operon of E.coli produces three proteins that import disaccharide lactose,breaking it into two monosaccharides, glucose and galactose The activator switch –lac operon has two regulatory sites CAP site adjacent to lac promoter –Ensures genes not transcribed when glucose is present –If glucose absent, high levels of cAMP in cell cAMP binds to CAP,CAP binds to DNA, promoter functional –If glucose present, levels of cAMP are low CAP prevented from binding to DNA, lac promoter not functional The repressor switch –Operator is second regulatory site, adjacent to promoter fig –lac repressor binds to operator, only when lactose absent –Repressor covers part of promoter when bound to operator Result: lac operon only expressed when glucose not present, and lactose present

Transcription control in eukaryotes is more complex Cells prefer constant conditions - gene expression help control homeostasis Changes in gene expression produce variety of results –Compensate for changes in body's physiological condition –Ensure that correct genes are expressed in development Changes in gene expression serve needs of whole, not individual cell

Control ‘at a distance’ Eukaryotes make use of transcription factors, complex multi-protein molecules that cause DNA to loop. Therefore, blocking of regulatory proteins at some distance down a DNA sequence may effect a gene’s expression - may involve ‘enhancers’ Binding of transcription factor begins at, but is not limited to, the TATA box Transcription inhibited by –Anything that reduces availability of any factor –Anything that limits its ease of assembly into complex In vertebrates, methylation may be involved

Post-transcriptional control Gene splicing removes introns Possible control of transport of mRNA out of nucleus Translation repressor proteins (prevent binding of ribosome) Selectively degrading mRNA transcripts (e.g., growth factor transcripts are notoriously unstable)