Practice question

Sigma70 comprises domains sigma1.1 through sigma4 connected by linkers. Region 3.2, approximately amino acids 510-520 forming acidic loop between domains sigma2 and sigma4, threads through RNA polymerase active site cleft into RNA exit channel. It acts as a molecular mimic of nascent RNA, positioning initiating nucleotides and stabilizing abortive initiation complexes while preventing extension beyond five to six nucleotides. When RNA grows longer, it collides sterically with this loop, necessitating ejection. Removal of 3.2 linker clears exit path and permits promoter clearance and transition to stable elongation.