Mutation-induced changes in RNA polymerase-lac ps promoter interactions.

A composite rate assay has been applied to investigate the rate and mechanism of formation of open promoter complexes. Seven DNA templates were studied which were related to the lac ps promoter by single base pair changes in the -10 or -35 region of promoter sequence homology. These small changes induce a nearly 3 order of magnitude variation in the rate of open complex formation. This variation persists over a wide range of concentrations of RNA polymerase. Nevertheless, all promoters direct open complex formation which proceeds through a "closed" or A polymerase:DNA complex which dissociates readily. These data, when taken together with our previous results on the lac "spacer" mutations, demonstrate that mutation of the lac ps promoter leads to changes in the rate of open complex formation predicted by the following rule. Changes which substitute a less conserved element of sequence in the -10 and -35 regions, or of length in the spacer, always decrease the rate in this homologous series of promoters.