Sequence of the ebgA gene of Escherichia coli: comparison with the lacZ gene

We have sequenced the ebgA (evolved beta-galactosidase) gene of Escherichiacoli K12. The sequence shows 50% nucleotide identity with the E. coli lacZgene, demonstrating that the two genes are related by descent from a commonancestral gene. Comparison of the two sequences suggests that the ebgA genehas recently been under selection. A significant excess of identical,rather than synonymous, codons used to encode identical amino acids at thesame positions in the aligned sequences implies that some form of selectionis operating directly at the DNA level. This selection is independent of,and in addition to, selection based on codon usage or on function of thegene products.