Capsulation gene loss and 'rescue' mutations during the Cap+ to Cap- transition in Haemophilus influenzae type b.

Genes for Haemophilus influenzae type b capsule expression are duplicated to form a potentially unstable structure, cap, of directly-repeated chromosomal regions of approximately 17 kb. Capsule-deficient mutants arise in a two-stage process, initiated by rec-dependent reduction of this region from two copies to one. This recombinational event is usually lethal, only about 1/200 surviving to form slow-growing colonies of organisms that continue to synthesize polysaccharide but are defective in its export. A variety of secondary 'rescue' mutations within cap can occur to reduce polysaccharide synthesis and restore normal organism appearance and colony morphology.