Same sequence of bases is seen 53 along both strands Why dimers and oligomers o

Same sequence of bases is seen 53 along both strands

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Same sequence of bases is seen 5’→3’ along both strands Why dimers and oligomers? o improved specificity & stability Inducible Promoters Key Concept: The primary reason for gene regulation in bacteria is to respond to changes in the environment, such as nutrient availability. Regulation of the rate transcription initiation is the most important step determining whether a gene is expressed. Regulated genes/operons equipped w/ inducible promoter! (can be turned on/off) Activators are transcription factors that increase rate of transcription from a promoter Repressors are transcription factors that decrease rate of transcription from a promoter . In bacteria , both activators and repressors have helix-turn-helix DNA binding domains. Positive Regulation Activator requires ligand “inducer” to bind DNA Ligand prevents activator from binding to DNA Activators recruit RNA polymerase to a promoter. Negative Regulation Repressor requires ligand to bind DNA Ligand “inducer” prevents repressor from binding to DNA. Repressors inhibit transcription by RNA polymerase! Regulatory Elements Key concept : Bacterial promoters = inherently in active state available for transcription most prokaryotic gene regulation occurs as a result of repressor binding to a site proximal to an inducible promoter Repressor binds to an operator either within the promoter or downstream of the promoter o Repressor bound atop promoter sequence blocks RNA polymerase from binding to promoter o Repressor bound downstream inhibits promoter clearance! Only want repressor bound in beginning (upstream) to block transcription Key concept : Some prokaryotic gene regulation results from binding of an activator to a site proximal to an inducible promoter. o Activator binds to positive regulatory element located upstream of a promoter o Activator recruits RNA polymerase to a weak promoter .
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Structural Genes of the lac Operon All 3 genes in the operon function in lactose metabolism Glucose = preferred carbon source for E. coli consumed ahead of lactose o In the absence of glucose, adenylate cyclase produces cAMP . -galactosidase catalyzes two reactions: o Lactose (12C) → galactose (6C) + glucose (6C) o Lactose (12C) → allolactose (12C) Regulation in the lac Operon Lac operon has 3 genes : lacZ, lacY, lacA (ZYA) Negative regulation o lacI gene is located upstream of the lac operon and encodes the lac repressor lac repressor binds the operator in the absence of allolactose (or IPTG) operator site is located at the transcription start site Positive regulation o cAMP receptor protein (CRP) binds DNA in the presence of its inducer, cAMP o CRP binding site located upstream of lac promoter lac Repressor is a Tetramer 4 identical subunits: a “homotetramer” forms a “dimer of dimers” o All 4 subunits have helix-turn-helix motifs for sequence-specific binding to 2 separate palindromic operator sites o
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  • Spring '08
  • Biochemistry, dna polymerases

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