The Genetics of Encapsulation in Haemophilus influenzae

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Abstract
In Haemophilus infiuenzae typeb (Hib) strains the cap locus with very few exceptions contains an unstable direct repeat of ∼ 17 kbofDNAflanking an ∼ 1-kb bridge region containing the gene bexA. Each repeat contains genes necessary for polysaccharide synthesis, export, and surface expression,with BexAa 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 lS1016 flank the genecluster. 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 wayand becomefixedin the population through deletionof one copy of bexA.

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