Spacer mutations in the lac ps promoter.

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Abstract
Mutations have been constructed that delete either 1 or 2 base pairs near position -19 in the [Escherichia coli] lac ps promoter. Deletion of either of 2 adjacent base pairs increases the rate of open complex formation by nearly an order of magnitude. Two promoters that have different single-base deletions are indistinguishable by either their rates of open complex formation or stability of the open complexes once formed. However, simultaneous deletion of both base pairs produces a promoter that forms complexes at a rate similar to that of the unmodified DNA sequence. The maximal rate of open complex formation is achieved at a spacer length of 17 base pairs, the most frequently occurring spacer length among promoters. The spacing between the 2 strongly conserved regions of sequence homology is apparently an important determinant of the rate of open complex formation. A model is suggested that proposes that 3 important promoter elements, the -10 region, the -35 region and the spacer region, act simultaneously to facilitate open complex formation by RNA polymerase.