SYNONYMOUS SUBSTITUTIONS ARE CLUSTERED IN ENTEROBACTERIAL GENES

The spatial distribution of synonymous substitutions in enterobacteria l genes is investigated. It is shown that synonymous substitutions are significantly clustered in such a way that a synonymous substitution in one codon elevates the rate of synonymous substitution in an adjace nt codon by about 10%. The level of clustering does not appear to be r elated to the level of gene expression, and it is restricted to a rang e of two or three codons. There are at least three possible explanatio ns: (1) sequence-directed mutagenesis, (2) recombination, and (3) sele ction.