The distributions of nucleotides near bacterial transcription initiation and termination sites show distinct signals that may affect DNA geometry.

Compilation and analysis of all bacterial sequences which are aligned by their transcription initiation sites show a dramatic behavior of the four nucleotides. Large peaks of T and A are observed. This highly nonrandom distribution is likely to affect the DNA geometry in addition to affecting the strength of binding between the two DNA strands. Following this site, the G and C rise above their overall bacterial mean. Alignment by transcription termination sites indicates that this behavior continues till the mRNA 3' termini. At this site the concentrations of A and T rise again above the mean. Analysis of the distributions of the 256 quartets in the 1000 nucleotide regions surrounding both transcription initiation and termination sites has been carried out. Some A/T combination sequences may serve as signals to the bacterial transcription machinery, in addition to the well-established TTGACA and TATAAT at positions -35 and -10, respectively, and a run of Ts at the transcription termination site. The frequent occurrences of (dA)/(dT) runs in the vicinity of these sites may result in curved DNA structures, affecting recognition and the nature of the interaction between the RNA polymerase and the DNA.