LOW SYNONYMOUS SITE VARIATION AT THE LACY LOCUS IN ESCHERICHIA-COLI SUGGESTS THE ACTION OF POSITIVE SELECTION

We have determined the nucleotide sequences of seven lacY alleles isol ated from natural isolates of Escherichia coli. Nucleotide heterozygos ity estimates for this locus were compared to those obtained from prev ious studies of intraspecific variation at chromosomal loci, revealing that lacY has unusually low synonymous site variation. The average pa irwise heterozygosity of synonymous sites (Ks = 0.0112 +/- 0.0100) is the second lowest reported and the lowest for loci that have an equiva lent level of nonsynonymous variation. We consider several hypotheses to explain how different forces in evolution could act to create the o bserved pattern of polymorphism, including selection for translational efficiency and positive selection. Our analysis most strongly support s the hypothesis that positive selection has acted on the lacY locus i n E. coli.