CHAPTER 1. The Transition from Transcription Initiation to Transcription Elongation: Start-site Selection, Initial Transcription, and Promoter Escape

In transcription initiation, RNA polymerase (RNAP) binds to promoter DNA, unwinds a turn of promoter DNA to yield an RNAP–promoter open complex containing an unwound “transcription bubble,” and selects a transcription start site (TSS). In the next step of initiation, termed “initial transcription,” RNAP remains bound to the promoter and synthesizes an RNA product of a threshold length of approximately 11–15 nucleotides. In the final step of initiation, termed “promoter escape,” RNAP breaks free of the promoter to yield a transcription elongation complex that synthesizes the rest of the RNA product. As a result of research over the last two decades, we now have a detailed mechanistic understanding of TSS selection, and we now understand broad outlines of initial transcription and promoter escape. Here we review the current understanding of TSS selection, initial transcription, and promoter escape, focusing on these processes as they occur in the best characterized example, transcription initiation by Escherichia coli RNAP-σ70 holoenzyme, but also summarizing these processes as they occur in eukaryotic RNAP I, II, and III.