The consequences of base-pair substitution mutations in AT- and GC-rich bacteria

The likely consequences, in terms of premature stop codons, detectable missense mutants, silent missense mutants, and degenerate codon changes, have been determined for all 12 individual base substitution changes. This has been done for the full, 61 sense codon, genetic code and also for the much more limited codon availabilities of AT- or GC-rich DNA. The specificities and outcomes of individual base substitutions are likely to be rather different at AT- or GC-rich extremes, and also from the situation at an intermediate DNA base-ratio where all 61 sense codons are available. In particular, at DNA base-ratio extremes many mutations will be to non-utilized codons, which may well act as nonsense mutants. These in turn will give novel classes of suppressor-containing revertants. Even in bacteria with intermediate DNA base-ratios, particular codons for a given amino acid may be favoured, over alternatives, because their use maximizes, or minimizes, the mutational consequences of one, or more, base substitution changes.