Regulation of Gene Expression Chapter 18 Regulation of Gene Expression

Overview: Gene Expression Prokaryotes & eukaryotes use different methods to turn genes “on” or “off” Ultimately gene expression is about efficient use of resources/energy If you don’t need it, don’t make it © 2011 Pearson Education, Inc.

Operons: The Basic Concept Controls transcription An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control © 2011 Pearson Education, Inc.

Parts of the operon Promoter (RNA Polymerase binding site) Genes Operator (Repressor binding site) Genes

Polypeptide subunits that make up enzymes for tryptophan synthesis Figure 18.3a trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Start codon Stop codon 3 mRNA 5 mRNA 5 E D C B A Protein Inactive repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on

Other players: Repressor: A protein that can turn the operon “off” or “on” by binding to the operator, blocking RNA polymerase regulatory gene: makes the repressor (usually “upstream” from the operon. Co-repressor: A molecule that binds to the repressor protein to change its shape © 2011 Pearson Education, Inc.

Tryptophan (corepressor) Figure 18.3b-1 DNA mRNA Protein Active repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

Figure 18.3 trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Start codon Stop codon 3 mRNA 5 mRNA 5 E D C B A Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on DNA No RNA made Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

Repressible and Inducible Operons: Negative or Positive Feedback? Repressible operon: default is “on;” binding of a repressor shuts off transcription (ex: trp operon) inducible operon default is “off”; a molecule called an inducer inactivates the repressor and turns on transcription (ex. lac operon) © 2011 Pearson Education, Inc.

Figure 18.4 Regulatory gene Promoter Operator DNA DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacI lacZ lacY lacA RNA polymerase Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. 3 mRNA mRNA 5 5 Protein -Galactosidase Permease Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

When do we see inducible vs. repressible feedback? Inducible enzymes usually in catabolic pathways; Repressible enzymes usually function in anabolic pathways © 2011 Pearson Education, Inc.

Eukaryotic gene expression is regulated at many stages Chromatin modification Transcription RNA Processing Transport to cytoplasm Degradation of mRNA Protein processing Degradation of protein Transport to cellular destination © 2011 Pearson Education, Inc.

Figure 18.6 Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation DNA Gene available for transcription Gene Transcription RNA Exon Primary transcript Intron RNA processing Tail Cap mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells. Polypeptide Protein processing, such as cleavage and chemical modification Active protein Degradation of protein Transport to cellular destination Cellular function (such as enzymatic activity, structural support)

Differential Gene Expression Cell differentiation results from gene expression, Abnormalities in gene expression can lead to diseases including cancer © 2011 Pearson Education, Inc.

Regulation of Chromatin Structure Histone proteins, part of Chromatin are modified to allow for gene expression 1) Histone acetylation: acetyl groups are attached to positively charged lysines in histone tails 2) Phosphorylation: phosphate groups added next to methyl groups This “loosens” the structure of the chromatin and allows for transcription. (See video: DNA Packing) © 2011 Pearson Education, Inc.

Amino acids available for chemical modification Figure 18.7 Histone tails DNA double helix Amino acids available for chemical modification Nucleosome (end view) (a) Histone tails protrude outward from a nucleosome Figure 18.7 A simple model of histone tails and the effect of histone acetylation. Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription

DNA Methylation- reduces transcription addition of methyl groups, can cause long-term inactivation of genes in cellular differentiation Phosphorylation counteracts methylation © 2011 Pearson Education, Inc.

Epigenetic Inheritance Although the chromatin modifications just discussed do not alter DNA sequence, they may be passed to future generations of cells The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance © 2011 Pearson Education, Inc.

Regulation of Transcription Initiation Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery © 2011 Pearson Education, Inc.