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Lecture 13

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Biology 1002B
Denis Maxwell

Lecture 13: Prokaryotic Gene Regulation 1. Identify the main features of bacterial operons  Operon (promoter – operator – genes) o Cluster of prokaryotic genes and the DNA sequences involved in their regulation o Transcription unit: cluster of genes transcribed into single mRNA o Usually proteins encoded by genes in the same operon catalyze steps in same process o Coordinate system to turn all genes on or off o Bring multiple genes under control under single promoter o Many are controlled by more than one regulatory mechanisms  Operator – short binding regulatory DNA sequence in operon to turn on/off gene  Repressors/activators control 1 ≤ operon o Repressor – reduces transcription o Activator – increases transcription 2. E. Coli metabolism of lactose: inducible lac operon  Constitutive (always expressed) gene lacI o Gene separate from lac operon make regulatory protein, lactose repressor  lacZ – make enzyme ß-galactosidase to catalyze disaccharide lactose  glucose + galactose o Metabolized by other enzymes, producing energy for glycolysis and Kreb’s Cycle  lacY – make permease enzyme that transports lactose actively into the cell  lacA – make transacetylase enzyme to metabolize acetyl-CoA’s acetyl transfer to β-galactosidase  No lactose (normal) o Lac repressor electrostatically (+ and – charges on double helix; not base pairing) bind to operator  Often proteins that bind to DNA bind as a dimer and stabilizes looped out DNA  Block RNA Polymerase from binding to promoter and transcribing  When lactose is present, lac operon is turned on o Lactose + low levels of ß-galactosidase  allolactose (isomer of lactose) o Allolactose is an inducer (help increase expression) for lac operon  Binds to lac repressor and alter its conformation so it can no longer bind to operator  RNA polymerase can then bind to promoter upstream of laz Z and start transcription  Quick turnover permits cytoplasm to be cleared quickly of the mRNAs transcribed from an operon o Each step is catalyzed by an short life enzyme that is coded by a gene o When lactose is used up, there is no allolactose so it does not inactivate repressor 3. DNA signals in RNA-coding genes/what signals would you find in the Trp tRNA gene?  Only promoter and terminator DNA Non Template (Codon) 5' > T G G> 3'  Not translated since there is no SD box, Template (Anti-codons) 3' < A C C < 5' s
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