EN
In the open transcription complex (RPo), Escherichia coli RNA polymerase o and a70 subunits are known to be in contact with each other and with the promoter region overlapping the -35 hexamer and the proximal part of the UP element. To probe the effect of An DNA bending tracts in this region on initiation of transcription, kinetics of the formation of RPo by Escherichia coli RNA polymerase at two groups of synthetic consensus-like promoters bearing single DNA bending tracts (i) A5 within the proximal subsite region of the UP element (promoters Pk and Pl) and (ii) A5 (Pg) or A8 (Pm) in the region including the downstream end of the proximal UP subsite and the -35 consensus hexamer was studied in vitro using the fluorescence-detected abortive initiation assay. The kinetic data obtained demonstrate that the overall second-order rate constant ka of RPo formation is: (i) by almost one order of magnitude larger at Pk and Pl, relative to that at a control unbent promoter, and mainly due to a higher value of the equilibrium constant, Kj, of the initial closed complex; and (ii) several-fold smaller at Pg and Pm owing to a strongly decreased value of K^. For Pm, the latter parameter was found to be dependent exponentially on four Mg ions, as compared with the seven ions remaining in equilibrium with the initial closed complex at the parent Pa promoter. This indicates that promoter region bearing a stiff A8 T8 fragment of B'-DNA forms a smaller number of ionic contacts with the a subunit. These findings provide a new insight to and support the present model of interactions between RNA polymerase a and o subunits with the proximal UP subsite and the -35 region of promoters.