Nucleotides downstream of start codons show marked non-randomness in Escherichia coli but not in Bacillus subtilis.

This study aimed at measuring the nucleotide non-randomness in the region downstream of start codons in bacterial genes and to see if the non-randomness differs between biased and unbiased genes, in terms of the effective number of codons (Nc) and the codon adaptation index (CAI). In Escherichia coli, there was a marked elevation in nucleotide conservation for the genes having low Nc-values compared to the genes having high Nc-values, i.e the more biased genes showed a higher level of non-randomness. Likewise, the genes displaying high CAI-values showed stronger nucleotide conservation than the genes of low CAI-values. This elevated conservation is visible up to approximately 15-17 nucleotides downstream of the start codon, after which there is little difference. This indicates that there may be distinct selectional mechanisms acting upon the first 5-6 codons within genes in E. coli. In B. subtilis, these effects are less pronounced, if present at all. Furthermore, analyses of codons used in this region were not in support of the hypothesis that the elevation in nucleotide non-randomness is a question of selection for certain optimal codons.