Secretion and expression of the Pasteurella haemolytica Leukotoxin

The Pasteurella haemolytica leukotoxin gene cluster (lktCABD) is homologous to the Escherichia coli hemolysin locus (hlyCABD). Since the cloned leukotoxin (LktA) is not secreted from E. coli cells, a heteroplasmid complementation system was developed that permits secretion of the leukotoxin from cells expressing the hemolysin transport proteins HlyB and HlyD. We observed that the secreted leukotoxin protein had weak hemolytic activity when activated by either the HlyC or LktC proteins and that LktC expressed in E. coli could confer weak hemolytic activity upon hemolysin. Thus, it appears that the accessory proteins of the leukotoxin and hemolysin gene clusters are functionally similar, although their expression in E. coli is not equivalent. Northern (RNA) blot analysis of the P. haemolytica leukotoxin gene cluster revealed a major 3.5-kilobase transcript that includes the lktC and lktA genes. The start site for this transcript mapped to a cytosine residue 30 nucleotides upstream from the putative start of lktC; a similar initiation site was observed in E. coli, although adjacent cytosine and adenine residues were also utilized. The 3.5-kilobase transcript terminated near the rho-independent terminator structure between lktA and lktB, but transcription may continue, via antitermination or de novo transcription initiation, into the downstream lktB and lktD genes. We propose that the lack of LktB and LktD function in E. coli is a result, at least in part, of poor lktBD transcription and suggest that a P. haemolytica-specific regulator is required for optimal expression of the leukotoxin genes.