Molecular characterization of Bordetella bronchiseptica filamentous haemagglutinin and its secretion machinery

Two closely related pathogens, Bordetella pertussis and Bordetella bronchis eptica, share a number of virulence factors. Filamentous haemagglutinin (FH A) is widely regarded as the dominant adhesin of B. INTRODUCTION pertussis, and its multiple binding activities have been well characterized. This lar ge protein is produced and secreted at high levels by B. pertussis and sign ificantly lower levels by B. bronchiseptica strains. FHA secretion is media ted by a single outer-membrane accessory protein, FhaC. The genes encoding FHA and FhaC in B. bronchiseptica were characterized by sequencing and func tional analyses and are highly similar to those of B. pertussis. The most d istinctive feature of B. bronchiseptica FHA is additional repeats in the N- terminal portion of the predicted protein. Interestingly, a point mutation in the fhaB promoter region of the B. bronchiseptica GP1 isolate, relative to other isolates, was found to be detrimental to promoter activity and to FHA production. FhaC and the N-terminal secretion domain of FHA of B. bronc hiseptica were fully functional for secretion in B. pertussis. Thus, the di fferent levels of FHA secretion by these Bordetella species might reflect d ifferences in physiology, composition and structure of cell envelope, or di fferential protein degradation. Characterization of FHA expression and func tion may provide clues as to the basis of host species tropism, tissue loca lization and receptor recognition.