Practice question

UP element is AT-rich sequence located approximately -40 to -60 upstream of transcription start site, found in strong rRNA and tRNA promoters. Crystallographic studies show C-terminal domain of α subunit, α-CTD connected via flexible linker, contains helix-hairpin motif binding minor groove of UP element AT tracts, providing additional anchoring energy independent of sigma -10 and -35 contacts. Interaction can enhance transcription up to thirty fold. β, σ70, β', ω subunits do not directly contact UP. Presence explains why some promoters lacking perfect consensus still strong due to extra α-CTD mediated recruitment of polymerase holoenzyme.