week_8_lecture_2

Binding stimulates stimulates transcription from prm

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Unformatted text preview: h binding partial symmetry about central base pair central central bp divides each central site into 2 half-sites, which show similarity to consensus sequence each half-site binds each one monomer in repressor dimer repressor site 1 in each operator site has a 10X greater affinity for λ repressor than other sites than λ LYSOGENY: AUTOGENOUS CIRCUIT LYSOGENY: -we we end up with 4 monomers of repressor at each operator operator • Unlike PL & PR , Unlike repressor binding stimulates stimulates transcription from PRM • mutations identified in mutations repressor that prevent transcription from PRM; known as positive control mutations & identify region of repressor that may directly interact with RNAP Figure 18-25 Figure 18-26 Figure 18-28 DNA looping is involved in repression of PRM by cI • The 4 monomers of CI The repressor bound to each operator can form an octamer, causing formation of DNA loop formation • When cI reaches high When levels this loop can enable binding of cI at OR3 and OL3, leading to and 3, repression of PRM RM INTERGENIC SUPPRESSION BETWEEN λ REPRESSOR & RNAP • Compensatory intergenic Compensatory suppressor mutations in RNAP can suppress repressor mutants suppress defective in PRM transcriptional activation - confirms that RNAP & cI repressor directly interact at PRM THE BATTLE OF cI vs. cro THE cI vs. cro • • • • • Weaver 2002 Molecular Biology Ed. 2 • • If cI establishes control then If cI lysogeny lysogeny If cro establishes control then lysis If cro cro wins by blocking PRM transcription & by repressing early genes dispensable for lytic cycle genes cro same operator as cI (λ repressor) but with reverse affinity: cro affinity OR3 > OR2 > OR1 cro Cro binds OR3 first and thereby Cro blocks RNAP binding at PRM - no RM maintenance of λ repressor repressor Cro subsequently “fills up” OR & Cro OL preventing cII & cIII gene cII cIII expression - no λ repressor expressed from PRE expressed all routes for synthesis of λ all repressor are blocked and lytic cycle assured cycle THE BATTLE OF cI vs. cro: THE cI vs. DETERMINED BY CII? DETERMINED • as [CII] increases, lysogeny increases • CII sensitive to proteases and protected from CII these by CIII these • Cellular proteases that degrade CII are higher Cellular when cells grown in rich medium, lower during starvation starvation • CII degradation & l...
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