The bootstrap tree was inferred by the NJ method with JTT model and gamma-distributed rates. Homologous bacterial PBPs were used as an outgroup to root the trees. Bootstrap values of more than 70% were presented on the node. The accession numbers and corresponding database names (gi for GenBank, sp for Swiss-Prot) for each sequence are placed in square brackets. The total numbers for each class are put in parenthesis followed by the class name.