Distribution and functional significance of A/T tracts in promoter sequences of Escherichia coli

Following earlier publications that revealed specific distribution of A/T tracts in the early transcribed regions, the disposition of these elements was studied in a wider region of promoter DNA from position -250 to +150 relative to the transcription start point. A set of preferred positions of A/T tracts distributed at a shorter distance one from another than in the early transcribed regions (1 and 1.5 helix turns, respectively) were found upstream of the -35 element. Such a positional dependence allows speculating that the A/T tracts of the two regions differ functionally at certain stages of the transcription cycle. The influence of these elements on the local properties of elongation complexes was examined using T7D promoter mutant derivatives devoid of A/T tracts in two critical positions (+41 and +57) and single-round transcription assays. We found that elimination of A/T tracts resulted in appearance of correspondingly shortened RNA products. Specific disposition of A/T tracts in the regions surrounding promoter sites may, therefore, play a significant role in the functioning of the transcription complex.