Molecular analysis of a gene encoding a serum-resistance-associated 76 kDa surface antigen of Haemophilus somnus

Haemophilus somnus is a Gram-negative bacterial bovine pathogen which can cause disease or be carried asymptomatically. We previously showed that four serum-sensitive isolates from asymptomatic carriers lacked a 13.4 kb sequence of chromosomal DNA that was present in two virulent serum-resistant strains. We have since sequenced 5 kb of the 13.4 kb fragment from a serum-resistant strain, which contained an open reading frame (ORF) of at least 4.5 kb. From Western blot analysis, the ORF was shown to encode a 76 kDa protein (p76) that co-migrated with a 76 kDa H. somnus surface protein. Both the recombinant and natural p76 reacted with convalescent-phase serum from a cow in an experimental H. somnus abortion study. The translational start site for p76 was identified by deletion analysis of subclones of the 5 kb cloned sequence. The 4.5 kb ORF contained 1.2 kb tandem direct repeats (DRs), with 65% identity between the two repeats at the protein level. The 5' DR (DR1) included the start site for the 76 kDa protein, and DR2 had a flanking inverted repeat, suggestive of an insertion-sequence-like element.