Functional characteristics of the rrnD promoters of Escherichia coli

The function of the tandem rrnD promoters (P1, P2) of Escherichia coli, which are highly efficient in directing rRNA synthesis, was studied in vitro using the strong hybrid promoter PtacI as a reference. One of the characteristics of the rrnD promoters is a pronounced instability of binary and initiating complexes formed with RNA polymerase. The rate of productive complex formation and of chain initiation at these promoters was found to be limited by a step in binary complex transitions with an apparent first-order rate constant equal to 3.9 x 10(-2) s-1. A comparison of this rate with that determined previously by filter binding assays (Gourse, R. (1988) Nucleic Acids Res. 16, 9789-9809) suggests that the rate-limiting step is a conversion of an intermediate species of open complex to one that is efficient in productive initiation. The slow rate of this reaction and the instability of open complexes account for the relatively low competitive strengths of the rrnD promoters. However, this limitation of rrn promoter function changes with promoter occupancy because the rate of chain initiation increased after completion of the first round of initiation. Despite their poor competitive strength, the rrnD promoters are more productive than PtacI at nonlimiting RNA polymerase concentrations. This can be ascribed to the different rates with which RNA polymerases leave PtacI and the rrnD promoters. These functional differences of the promoters are consistent with a "stressed intermediate" model of chain initiation (Straney, D.C., and Crothers, D.M. (1987) J. Mol. Biol. 193, 267-278) which predicts that rapid clearance of the rrn promoters is mechanistically related to the instability of the binary complexes.