Abortive initiation refers to synthesis of RNA of length
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
Ref: Watson et al. Chapter 13: Abortive initiation RNA length ≤10 nucleotides; Alberts Chapter 6 Abortive transcripts initiation complex instability