EXPRESSION AND PHYLOGENETIC-RELATIONSHIPS OF A NOVEL LACZ HOMOLOG 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 Delta lac mutant. The A. pleur opneumoniae lacZ gene has an open reading frame of 3015 bp which could encode a protein with a predicted molecular mass of 117 022. The dedu ced protein shares 26.8-34.8% identity with beta-galactosidases from b oth Gram-positive and Gram-negative bacteria. Sequences with homology to seven regions commonly found in beta-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 th e place of Gly-794, a residue which has been implicated in substrate r ecognition. The sequences flanking the A. pleuropneumoniae lacZ gene d o not share homology with known transport or regulatory genes nor do t hey share homology with cAMP receptor protein (CRP) or LacI binding si tes. Low levels of beta-galactosidase activity could be detected when the protein was expressed from a multicopy plasmid in E. coli Delta la c and when it was measured in A. pleuropneumoniae. The level of activi ty was not markedly reduced in the presence of glucose. Although the A . pleuropneumoniae LacZ shares some features with other beta-galactosi dases, its constitutive expression and an unusual active site residue suggest that it may have a unique function.