STEP1 INITIATION A. Formation of a closed promoter complex • RNA polymerase σ factor (subunit) recognizes/binds to CORE PROMOTER special -35 and -10 boxes ( consensus sequences) upstream of the transcription start site • If there’s a UP element , it’s recognized by the α-subunit’s C-terminal domain (CTD) o UP elements aren’t essential for transcription initiation, but make the promoter stronger • DNA is still in “closed” double strand conformation = loosely associated with RNA polymerase B. Formation of an open promoter complex • RNA polymerase melts a short DNA segment around the +1 start site • DNA strands are separated and exposed in “open” conformation = tightly associated with RNA pol • DNA strands are “bent” Incorporate first few Nucleotides abortive Transcripts C. RNA polymerase forms the first few phosphodiester bonds • While still associated with the promoter region (i.e. consensus sequences) • May form several abortive transcripts (all the while still associated with DNA; RNA pol never falls off!) sigma cycle
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