Practice question

Within bacterial holoenzyme, sequence-specific readout of promoter consensus relies predominantly on sigma factor rather than core catalytic subunits. Sigma70 region 4.2 presents helix-turn-helix motif inserting into major groove at -35 element TTGACA, while region 2.4 interacts with -10 Pribnow box TATAAT and captures flipped bases to drive melting. α subunit C-terminal domain may touch UP element AT-rich tract, but -10 and -35 recognition is sigma intrinsic. β and β' subunits shape downstream DNA channel and catalytic Mg2+ but do not determine promoter consensus identity. Exchange of sigma alters promoter preference accordingly.