Expression and phylogenetic relationships of a novel lacZ homologue from Actinobacillus pleuropneumoniae

To learn more about the genetics and physiology of the important swine pathogen, Actinobacillus pleuropneumoniae , we cloned the lacZ gene by complementation of an Escherichia coli Δ lac mutant. The A. pleuropneumoniae lacZ gene has an open reading frame of 3015 bp which could encode a protein with a predicted molecular mass of 117 022. The deduced protein shares 26.8–34.8% identity with β‐galactosidases from both Gram‐positive and Gram‐negative bacteria. Sequences with homology to seven regions commonly found in β‐galactosidases are present and amino acids corresponding to active site residues Tyr‐503 and Glu‐537 in E. coli LacZ are also conserved; however, there is a leucine in the place of Gly‐794, a residue which has been implicated in substrate recognition. The sequences flanking the A. pleuropneumoniae lacZ gene do not share homology with known transport or regulatory genes nor do they share homology with cAMP receptor protein (CRP) or LacI binding sites. Low levels of β‐galactosidase activity could be detected when the protein was expressed from a multicopy plasmid in E. coli Δ lac and when it was measured in A. pleuropneumoniae . The level of activity was not markedly reduced in the presence of glucose. Although the A. pleuropneumoniae LacZ shares some features with other β‐galactosidases, its constitutive expression and an unusual active site residue suggest that it may have a unique function.