Bases immediate upstream of the TATAAT box of the sigma 70 promoter of Escherichia coli significantly influence the activity of a model promoter by altering the bending angle of DNA.

Different workers have found different bases of the spacer of the sigma 70 promoter of Escherichia coli to be important, depending on the base sequence of the two hexameric boxes of the naturally occurring promoter they were working on. Besides, there was no clue as to why particular bases worked better than others in particular positions. This necessitated a fresh look at the spacer region of a model promoter comprising all the consensus promoter elements. Randomisation of the three bases of the spacer in positions -15 to -13 with respect to the transcription initiation site, has elicited more than 50-fold variation in activity of the promoter, the highest and the lowest activities being 14,391(the three bases being GCA) and 264 Miller units (the three bases being AAA) respectively. Pairs of promoters of very similar activities were observed, even when the bases in these three positions were very different. The promoters with similar activities had similar three dimensional structures of the promoter DNA, as determined by molecular dynamics simulations. Randomisation of the three bases in positions -18 to -16 of the promoter that contained the triplet GCA in positions -15 to -13, resulted in promoters with highest activity of 15,759 (the triplet upstream of GCA being TAT) and lowest activity of 1,882 (the triplet upstream of GCA being AAA). Good correlation between the bending angles of the promoter DNAs and promoter activities could be observed, the R value being 0.8724. Retardation of electrophoretic mobility of the promoter DNAs correlated well with activity.