Practice question

During initial transcription, RNA polymerase undergoes iterative cycles of synthesizing and releasing short RNAs while remaining bound to promoter. Structural blockage occurs because sigma factor region 3.2 loop threads through RNA exit channel, obstructing extension beyond about nine to ten nucleotides. Polymerase synthesizes short oligoribonucleotides two to nine bases that escape into environment as abortive transcripts before productive escape. These ≤10 nucleotide products characteristic of abortive initiation phase. Overcoming this barrier requires conformational change displacing sigma block upon sufficient RNA length and energy, transitioning to elongation competent complex processively synthesizing full-length RNA.