Bacterial Genetics

1. Bacterial Genetics Regulation of Gene Expression

2. Bacterial metabolism • Bacteria need to respond quickly to changes in their environment • if have enough of a product, need to stop production • why? waste of energy to produce more • how? stop production of synthesis enzymes • if find new food/energy source, need to utilize it quickly • why? metabolism, growth, reproduction • how? start production of digestive enzymes

3. - Reminder: Regulation of metabolism • Feedback inhibition • product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway = inhibition

4. - Another way to Regulate metabolism • Gene regulation • block transcription of genes for all enzymes in tryptophan pathway • saves energy by not wasting it on unnecessary protein synthesis = inhibition

5. Gene regulation in bacteria • Control of gene expression enables individual bacteria to adjust their metabolism to environmental change • Cells vary amount of specific enzymes by regulating gene transcription • turn genes on or turn genes off • ex. if you have enough tryptophan in your cell then you don’t need to make enzymes used to build tryptophan • waste of energy • turn off genes which codes for enzymes

6. So how can genes be turned off? • First step in protein production? • transcription • stop RNA polymerase! • Repressor protein • binds to DNA near promoter region blocking RNA polymerase • binds to operator site on DNA • blocks transcription

7. Genes grouped together • Operon • genes grouped together with related functions • ex. enzymes in a synthesis pathway • promoter = RNA polymerase binding site • single promoter controls transcription of all genes in operon • transcribed as 1 unit & a single mRNA is made • operator = DNA binding site of regulator protein

8. RNA polymerase RNA polymerase repressor repressor promoter repressor protein operator Repressor protein model Operon: operator, promoter & genes they control serve as a model for gene regulation gene1 gene2 gene3 gene4 TATA DNA Repressor protein turns off gene by blocking RNA polymerase binding site.

9. RNA polymerase RNA polymerase repressor repressor repressor promoter repressor protein operator tryptophan tryptophan – repressor protein complex Repressible operon: tryptophan Synthesis pathway model When excess tryptophan is present, binds to tryp repressor protein & triggers repressor to bind to DNA • blocks (represses) transcription gene1 gene2 gene3 gene4 TATA DNA conformational change in repressor protein!

10. Tryptophan operon What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan binds allosterically to regulatory protein

11. RNA polymerase RNA polymerase repressor repressor repressor promoter repressor protein operator lactose lactose – repressor protein complex Inducible operon: lactose Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA • induces transcription gene1 gene2 gene3 gene4 TATA DNA conformational change in repressor protein!

12. Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose binds allosterically to regulatory protein

13. Jacob & Monod: lac Operon 1961 | 1965 • Francois Jacob & Jacques Monod • first to describe operon system • coined the phrase “operon” Jacques Monod Francois Jacob

14. Operon summary • Repressible operon • usually functions in anabolic pathways • synthesizing end products • when end product is present in excess,cell allocates resources to other uses • Inducible operon • usually functions in catabolic pathways, • digesting nutrients to simpler molecules • produce enzymes only when nutrient is available • cell avoids making proteins that have nothing to do, cell allocates resources to other uses

15. Figure 15.6a Signal NUCLEUS Chromatin Chromatin modification:DNA unpacking involvinghistone acetylation andDNA demethylation DNA Gene availablefor transcription Gene Transcription Exon RNA Primary transcript Intron RNA processing Tail mRNA in nucleus Cap Transport to cytoplasm CYTOPLASM

16. Figure 15.6b CYTOPLASM mRNA in cytoplasm Translation Degradation of mRNA Polypeptide Protein processing, suchas cleavage andchemical modification Active protein Degradationof protein Transport to cellulardestination Cellular function(such as enzymaticactivity, structural support)

17. Figure 15.7 Nucleosome Histonetails Unacetylated histones Acetylated histones

18. Figure 15.10-2 Promoter Activators Gene DNA Distal controlelement TATA box Enhancer General transcriptionfactors DNA- bendingprotein Group of mediator proteins

19. Figure 15.10-3 Promoter Activators Gene DNA Distal controlelement TATA box Enhancer General transcriptionfactors DNA- bendingprotein Group of mediator proteins RNApolymerase II RNApolymerase II Transcriptioninitiation complex RNA synthesis

20. Figure 15.11 Promoter Albumin gene Enhancer Controlelements Promoter Crystallin gene Enhancer (a) LIVER CELL NUCLEUS (b) LENS CELL NUCLEUS Availableactivators Availableactivators Albumin genenot expressed Albumin geneexpressed Crystallin genenot expressed Crystallin geneexpressed

21. Figure 15.11a (a) LIVER CELL NUCLEUS Availableactivators Albumin geneexpressed Crystallin genenot expressed

22. Figure 15.11b (b) LENS CELL NUCLEUS Availableactivators Albumin genenot expressed Crystallin geneexpressed

23. Figure 15.13 1 2 miRNA miRNA-proteincomplex The miRNA bindsto a target mRNA. mRNA degraded Translation blocked If bases are completely complementary, mRNA is degraded.If match is less than complete, translation is blocked.