The genetics of encapsulation in Haemophilus influenzae

In Haemophilus influenzae type b (Hib) strains the cap locus with very few exceptions contains an unstable direct repeat of approximately 17 kb of DNA flanking an approximately 1-kb bridge region containing the gene bexA. Each repeat contains genes necessary for polysaccharide synthesis, export, and surface expression, with BexA a critical component of the polysaccharide exporter. Only rare Hib strains have been identified in which cap lacks a direct repeat, though this is the norm for non-b serotypes. Examination of the ends of this single-copy locus shows that cap has the structure of a compound transposon: Copies of the insertion element IS1016 flank the gene cluster. This gives strains the capacity to amplify genes at cap by unequal homologous recombination. The cap duplication in Hib strains--subserving augmented production of polysaccharide--has apparently arisen in this way and become fixed in the population through deletion of one copy of bexA.