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2 comparison of the dna fit of trp repressor and

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Unformatted text preview: R an inactive trpR aporepressor aporepressor • High trp levels, aporepressor aporepressor activated by trp binding & repressor-corepressor binds operator operator • Lac operon --> inducer makes Lac repressor dissociate from operator operator • Trp operon --> corepressor Trp makes repressor associate with operator operator Weaver 2002 Molecular Biology Ed. 2 Comparison of the DNA fit of trp repressor and aporepressor with trp operator: Notice that the recognition helix of the repressor with the respect to the DNA major groove: pointing directly into the major groove with trp or downward without ATTENUATORS ATTENUATORS • Attenuator: intrinsic intrinsic terminator that stops transcription - common to many amino acid biosynthetic operons biosynthetic • E.g. deletion of upstream deletion DNA between trpE (the first trpE gene) & the operator increases trp operon trp expression independent of repression repression Weaver 2002 Molecular Biology Ed. 2 ATTENUATION Weaver 2002 Molecular Biology Ed. 2 • DNA between trpE & O includes attenuator and leader DNA trpE leader seq (trpL) encoding 14 amino acid peptide seq • The leader encodes 2 successive Trp codons - low Trp The causes ribosome stalling because of no tRNATrp causes • Ribosome stalling prevents terminator hairpin formation Ribosome prevents TERMINATOR vs ATTENUATOR HAIRPINs HAIRPINs • • • Leader-attenuator region RNA Leader-attenuator transcripts can form 2 different secondary structures secondary 1. High Trp: regions 1 & 2, and 3 & 4 and base pair forming classic terminator (stem-loop followed by run of U’s); (stem-loop ’s); this structure is more stable of two options options 2. Low Trp: regions 2 & 3 base pair no terminator forms (U’s too far no ’s from stem-loop) & transcription continues 1. 2. Weaver 2002 Molecular Biology Ed. 2 ATTENUATOR HAIRPIN Weaver 2002 Molecular Biology Ed. 2 • • • Low Trp: ribosome stalling prevents formation of two hairpin terminator structure as it blocks base pairing of regions 1 and 2 structure High Trp: leader translation continues to stop codon, ribosomes High dissociate & stable two hairpin terminator structure is formed; transcription ceases before reaching structural genes transcription Similar mechanism of attenuation used by his and phe operons: his Similar his phe his leader contains 7 His codons; phe leader 7 of 15 codons are Phe phe...
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