The molecular evolution of bacterial alkaline phosphatase: correlating variation among enteric bacteria to...

The phylogenetic distribution of the gene coding for bacterial alkaline phosphatase (phoA) was examined in nine species of enteric bacteria closely related to Escherichia coli. The nucleotide and protein sequences from the E. fergusonii and Serratia marcescens genes are presented. The spatial distribution of replaced amino acid residues in the aligned sequences is shown to be highly nonrandom and can be correlated with specific regions within the tertiary structure of the protein. There is an avoidance of replacements within the beta sheet of the protein, and there is an excess of replacements elsewhere, particularly in solvent-exposed residues. In addition, all positions across alpha helices do not accept replacements with equal frequency; there is a bias toward acceptance of replacements in the carboxyl ends of helices. To examine this further, mutations within the E. coli phoA gene were created using site-directed mutagenesis. The patterns seen from the sequence comparisons were verified in the laboratory-created mutants. The average activity of mutations within or near the beta sheet was approximately one-third of that within or near alpha helices, and multiple mutations within the carboxyl ends of alpha helices always possessed greater activity than did multiple mutations within the corresponding amino ends. The results indicate that identifiable regions within the protein are under different selective pressures and are therefore evolving at different rates.