Transcriptional control in prokaryotes

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  • 11 Transcriptionalcontrol in prokaryotes
    • Bacteria
      • Haploid; no gamete forming process. 'Crosses' can  be made between genetically distinct bacteria
      • Ecoli; mostly nonpathogenic, rapid growth rate, grows in simple liquids (broth) and on solid (agar) media
        • Minimal medium = simple carbon source (sugar), salt, trace minerals
          • Mutation that inactivates a biosynthetic pathway prevents growth on minimal medium
          • Complete medium contrains all amino acids, vitamins and nucleotides produced by biosynthetic pathways so even mutated  bacteria grow
      • Bacteria carrying mutation involved in a specific biosynthetic pathway will grow on minimal medium and the biosynthetic product
        • Inducing mutations and then growing on minimal medium will identify mutants. Changing composition can screen for genes involved in specific pathways.
      • Mutants that cannot synthesis essential nutrients are autotrophs. Strains that are wild type for a specific biosynthetic process are phototrophs
    • Gene transfer
      • Requires cell cell contract. The fertility factor - the F plasmid
      • Unidirectional; dna transferred across bridge
        • Donor strands contains a fertility factor F+. Means carries are able to conjugate with F- recipient cells
          • F plasmid transferred through the F-pilus 'Bacterial mating'
            • Plasmid has an orIT where transfer begins and tra genes which encode components of the transfer machinery
      • F factor can also cause transfer of host chromosomal genes from high frequency recombination (hfr) strains into F- strains
        • Hfr strains arise when the F plasmid recombines with host chromosomalDNA. F plasmid integrated. These then conjugate with F- cells. Hfr donor chromosome is nicked and transferred to recipient cell which is recombined into recipitent chromosome
          • Formation of hfr donor chromosomes allows the emergence of prototrophic colonies (requiring same nutrients) from a mixed culture of 2 different auxotrophic (inability to synthesise a product) mutants
    • Metabolic gene regulation
      • 1. Nutrient breakdown. Induction via nutrient e.g lactose. Enzyme encoding genes e.g. lac operon proteins. Breaksdown product e.g. to glucose. This repressed enzymes (negative feedback)
      • 2. Biosynthesis.substrate leads to enzyme encoding genes. Makes biosynthetic product wich represses enzyme. Feedback inhibition
      • The Trp operon; low trp - inactive trip repressor - trip synthesised; high trp - active trip repressor(rnapolymerase detaches) - transcription repressed
    • Lac operon
      • Operon = group of genes with closely related biochemical functions; multiple proteins synthesised from one mrna.
      • Induced in response to lactose substrate and encodes its breakdown. Blocked with glucose
        • Promoter, operator, B galactosidase gene, permease, transacetylase, terminator
      • Lac repressor binds to operator blocking transcription. Lactose causes it to dissociate allowing CAP-assisted transcription by rna polymerase. Production of glucose inhibits by inactivating rna polymerase cos-factor CAP protein


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