MOLECULAR EVOLUTION IN THE GND LOCUS OF SALMONELLA-ENTERICA

The gnd gene, the structural gene for 6-phosphogluconate dehydrogenase , was sequenced and analyzed in 34 isolates from different serovars of the seven subspecies of Salmonella enterica to provide comparative in formation on the evolution in this gene, which has been studied extens ively in Escherichia coli. The gene tree obtained by the neighbor-join ing method in general gave separate branches for each subspecies, with the few exceptions readily explained by recombination. There is evide nce of recombination involving transfer of long (more than 400 bp) and short (30-150 bp) segments of DNA. Four of the six long-segment trans fers detected are at the 5' end of the gene, and in all four cases a v ariant of the chi sequence is located close to the recombination junct ion and appears to have mediated the recombination events. We suggest that in these four cases and in a fifth case with intersubspecies tran sfer of the whole gnd gene, the adjacent rfb (O antigen) locus may hav e been transferred in the same event. The estimates of the number of s ynonymous substitutions per synonymous site, K-S, and the number of no nsynonymous substitutions per nonsynonymous site, K-A, within the E. c oli and S. enterica gnd genes, and also between the two species show a n interesting distribution, with K-S being lower toward the ends of th e gene and K-A in particular being lower in the first than in the seco nd domain, In S. enterica, synonymous sites also seem to be subjected to negative selection. The ratio of K-A to K-S was higher within S. en terica and E. coli than between them, which may indicate that intraspe cies variation is essentially between clones and that mildly deleterio us mutations can be fixed within clones, which would thus raise K-A wi thin species.