Kassavetis G A, Letts G A, Geiduschek E P
Department of Biology, Center for Molecular Genetics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0634, USA.
EMBO J. 1999 Sep 15;18(18):5042-51. doi: 10.1093/emboj/18.18.5042.
Transcription factor (TF) IIIB recruits RNA polymerase (pol) III for specific initiation of transcription. All three subunits of TFIIIB, TBP, Brf (the TFIIB-related subunit) and B", are required for transcription of supercoiled and linear duplex DNA, but we show here that B" is non-essential on a promoter that has been partly pre-opened by unpairing a short segment of the transcription bubble. These findings expose a striking similarity between transcriptional initiation by pol II, pol III and bacterial RNA polymerases: a preformed single-stranded DNA bubble upstream of the transcriptional start removes the dependence of pol II on TFIIE, TFIIH and ATP hydrolysis, and the dependence of pol III on B"; the favored placement of the transcription bubble for B"-independent transcription by pol III overlaps a DNA segment that interacts sequence specifically as single-stranded DNA with the sigma(70 )initiation subunit of Escherichia coli RNA polymerase holoenzyme.