Summary: Genes for type b capsule expression are duplicated to form a potentially unstable structure, , of directly-repeated chromosomal regions of approximately 17 kb. Capsule-deficient mutants arise in a two-stage process, initiated by -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 can occur to reduce polysaccharide synthesis and restore normal organism appearance and colony morphology.


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