Haemophilus influenzae is an important pathogen in children causing both invasive and mucosal diseases. The mechanisms of disease production in H. influenzae are undergoing intense study. This laboratory has identified a gene in H. influenzae with predicted amino acid homology to members of the ABC (ATP-binding cassette) transporter protein family. ABC transporters are important membrane proteins that move specific substrates across membranes via energy coupled to ATP hydrolysis. These proteins are involved in the pathogenesis of many bacteria, including other members of the family Pasteurellaceae. Nucleotide sequence analysis of a region of cloned chromosomal H. influenzae DNA revealed a 1.8kb open reading frame (htrA). The predicted amino acid sequence has significant homology to the hemolysin transporters HlyB ofEscherichia coli (31% identity) and LktB of Actinobacillus actinomycetemcomitans (32% identity) and the adenylate cyclase toxin transporter of Bordetella pertussis (33% identity). Additionally, an 894bp open reading frame immediately upstream of htrA was noted. The predicted amino acid sequence of this open reading frame (htrR) shares homology with the AraC family of transcriptional regulators. Polymerase chain reaction data reveals that htrA is present in both nontypeable and type b H. influenzae strains. Antisera is being generated to the C-terminal portion of the HtrA protein. In addition, three isogenichtrA mutants have been constructed in H. influenzae. The expression, cellular location and specific function of HtrA in H. influenzae will be characterized using the antisera and htrA mutants. In summary, a putative ABC transporter htrA, has been identified in H. influenzae in association with a regulatory gene, work is ongoing to define the specific substrate of the HtrA protein and its potential role in virulence.