Outer membrane protein P6 of Haemophilus influenzae binds to its own gene

Outer membrane protein P6 is an important antigen expressed on the surface of all strains of Haemophilus influenzae. The predicted amino acid sequence of P6 contains a region of alpha helices that shares sequence identity with a family of helix-turn-helix DNA-binding proteins. A search for sequence-specific binding sites that resemble an operator region within the gene revealed a sequence with striking homology to the consensus operator sequence for lambda Cro and repressor. To test the hypothesis that P6 binds its own gene, purified P6 on nitrocellulose was probed with plasmid DNA containing the P6 gene. P6 bound the P6 gene in this Southwestern blot assay. To further test the observation, gel shift analysis was performed. Gel shift assays using a P6-specific monoclonal antibody demonstrated that P6 in crude cell extracts binds to the region of the gene containing the putative binding site. Competition with a synthetic oligonucleotide corresponding to the putative binding site inhibited binding of P6 to the P6 gene on nitrocellulose and in the gel shift assay. In addition, this oligonucleotide bound directly to P6 on nitrocellulose. Finally, DNase footprinting confirmed that P6 bound specifically to the same region of the P6 gene. These results indicate that P6 binds to a sequence-specific site within its own gene, suggesting that P6 regulates its own expression. This represents the first example of a Gram-negative outer membrane protein binding to its own gene and has potentially important implications as a mechanism for regulation of expression of outer membrane antigens.