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.