The probability of nonsense mutation caused by replication-associated mutational pressure is much higher for bacterial genes from lagging than from leading strands.

We studied nucleotide usage biases in 4-fold degenerated sites of all the genes from leading and lagging strands of 30 bacterial genomes. The level of guanine in 4-fold degenerated sites (G4f) is significantly lower in genes from lagging strands than in genes from leading strands, probably because of the faster rates of guanine oxidation in single-stranded DNA leading to G to T transversions. The rates of cytosine deamination causing C to T transitions are also higher in lagging strands. We showed that the level of codons able to form stop-codons by the way of G to T transversions and C to T transitions is always higher than the level of codons able to form stop-codons by the way of C to A transversions and G to A transitions. This circumstance can be an explanation of the lower percent of ORFs in lagging strands of bacterial replichores than in leading strands.