DNA asymmetry and the replicational mutational pressure.

The mode of replication and organisation of bacterial genomes impose asymmetry on their nucleotide composition. The asymmetry is seen in coding and non-coding sequences and is reflected in the amino acid composition of proteins. The mechanisms generating asymmetry include: unequal mutation rates connected with replication and transcription, selection forces positioning genes and signal sequences nonrandomly in the genome, and protein coding constraints on coding sequences. There are different methods of visualising and measuring the asymmetry. Some of them can assess the contribution of individual mechanisms to the observed asymmetry and those have been described in greater detail. Asymmetric mutational and selection pressures differentiate the rates of evolution of genes on leading and lagging strands. The genes relocated to the opposite strand have to adapt to a different mutational pressure or are eliminated. Translocations from leading to lagging strands are more often selected against than from lagging to leading strands. Comparison of intergenic sequences that have lost the coding function to the original genes enables finding the frequencies of the twelve substitution rates in sequences free from selection. In the absence of selection, the half-time of substitution of a given type of nucleotide is linearly correlated with the fraction of that nucleotide in the sequence.