18 - Transcription (Prokaryotes); Regulation of the Operons

18 - Transcription (Prokaryotes); Regulation of the Operons...

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Remember: in prokaryotes gene regulation allows a single cell to adjust to its environment genes encoding catabolic enzymes are “off” when not needed and only turned “on” when the substrate is present Prokaryotic genes are organized into operons; also transcription and translation are coupled rapid response to changes in environment by rapid change in gene expression Example: after sensing lactose in their environment, bacteria make >50,000 copies of needed enzymes in 90 seconds!
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OPERON •Segment of the genomic DNA which consists of structural genes and adjacent regulatory sequence(s) •all structural genes in the operon code for proteins necessary in the same metabolic pathway regulatory gene(s) which are not part of the operon, are involved as well - code for regulatory proteins - important Catabolic operons Expression of enzymes involved in catabolism of energy source Biosynthetic operons (opposite from catabolic): Expression of enzymes involved in synthesis of energy source Francois Jacob and Jacques Monod in 1950s - In bacterial cell genes are organized in operons Fig 12.28 Karp
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•all structural genes are transcribed from the same promoter •they are under influence of same transcription factors (=regulatory proteins) that recognize the same regulatory elements •all structural genes in the operon are transcribed as part of the same (single) mRNA – polycistronic mRNA •regulatory genes have their own promoters… •lactose operon (catabolic), triptophan operon (biosynthetic), arabinose operon (catabolic)
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Gene expression in bacteria - some useful terminology/distinctions (similar in Eu): Constitutive genes Continuously expressed - required for “on going” survival of the cell - housekeeping genes (such as those required for protein synthesis, glucose metabolism/utilization etc.) Regulated - inducible genes Expression is regulated induced or suppressed – as needed, depending on the changes in environment
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The facts we are going to learn from lac (and other) operons
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18 - Transcription (Prokaryotes); Regulation of the Operons...

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