Gene Regulation Reading: Campbell Biology Chapter 18 Bacterial Genetics (pp. 337-341, not trp) Chapter 19 Eukaryote Gene Control (pp. 351-358) Learning Objectives: ¾Know why genes must be regulated ¾Describe the lactose (lac) operon as an example of bacterial gene regulation ¾Compare and contrast prokaryotic and eukaryotic gene regulation 1 Gene Expression DNA Î RNA Î Protein A powerful amplification process e.g., silk fibroin gene: Copies per silk gland cell 1 gene 104 mRNA molecules 109 protein molecules (in 4 days) 2 Enzymes ~ 103 conv / sec Gene Expression DNA Î RNA Î Protein Differentiation requires control All cells in an organism have the same genes ¾differentiation = different cellular activity ¾requires precise control Control of cellular activity = control of gene expression ¾operates at various stages in the process » transcription, transcript stability, translation, transport, protein stability, enzyme activity ¾most efficient to regulate early 3 Gene Regulation Why not delete the unnecessary genes? DNA is for ¾(1) storage, (2) transmission, (3) expression of genetic information Expression of different genes is needed ¾at different developmental stages ¾if surroundings change Regulation rarely involves loss of genes or changes to DNA 4 Regulating Transcription 5 Each gene (or bacterial operon) has its own promoter Regulatory proteins promote or inhibit binding of RNA polymerase to promoters Regulating Transcription e.g. the lac operon in Escherichia coli E. coli is a bacterium that lives in the gut It can use lactose (milk sugar) as food » this requires enzymes to metabolize lactose lactose permease β-galactosidase E. coli Lactose Lactose Glucose + Galactose (in gut) (in E. coli ) (Carbon & Energy Sources) » these enzymes Ï x103 in 15 min after supply of lactose (host drinking milk) ie ‘inducible enzymes’ Regulating Transcription lac operon: coordinated control The genes for lactose metabolism are: » clustered together on the chromosome » controlled by a single promoter » transcribed as a single transcript 1 promoter + several coding regions (cistrons) = an operon Allows coordinated control Regulating Transcription lac operon: negative control by repressor protein If lactose is absent, a regulatory protein (repressor) blocks lac transcription » by binding to an upstream ‘operator’ sequence » blocking RNA polymerase Transcriptional control: the bacterium does not waste energy making mRNA or enzymes for lactose metabolism in the absence of lactose Regulating Transcription lac operon: effect of an inducer (derepressor) If lactose is present, lac is transcribed » a lactose isomer binds to the repressor protein » the bound repressor protein changes shape » in this shape, it can not bind to the operator » RNA polymerase can now bind & transcribe Regulating Transcription lac operon: positive control by CRP + cAMP If glucose is abundant ¾lac activity is not needed CRP is a positive regulatory protein ¾activated by cAMP ¾assists RNA pol binding ¾Ï transcription rate If glucose is abundant ¾cAMP is scarce ¾little transcription of lac 10 Regulatory Proteins Key participants in transcriptional control Products of regulatory genes Often allosteric (shape determines activity) Recognize & bind specific DNA sequences Exert negative or positive effects Some genes are affected by multiple regulatory proteins (true for lac operon) Some regulatory proteins affect multiple genes 11 12 Eukaryotic Gene Regulation More genes, greater complexity: enhancers Similar in principle to prokaryotes » but no operons − allows multiple functions per gene? » coordination by shared regulatory elements Often multiple positive regulators / gene » multiple ‘enhancer’ sequences 13 Eukaryotic Gene Regulation More genes, greater complexity: enhancers Multiple positive regulators / gene allows ¾Fine control by cumulative effects ¾Combinatorial control » more patterns from fewer regulatory proteins − e.g. developmental patterns Eukaryotic Gene Regulation Greater complexity: control levels Transcriptional control » early, efficient » but slow (expression lag) Enzyme activation » expensive (~ waste energy) » but fast More intermediate opportunities in eukaryotes 14 » e.g. RNA processing & transport Regulatory Mutants Gene expression in the wrong place profoundly changes development » e.g. antennae like eyes » e.g. double flowers From sequence changes » in promoters / enhancers » in regulatory proteins Important determinants of evolution (e.g., multigene families) 15