Abstract

Nucleotide sequence analysis of a Shigella flexneri 2a virulence plasmid-borne locus revealed that it comprised three distinct genetic elements: a stretch of colicin la/lb-linked sequence, a truncated IS911 element, and a third element containing two ORFs that shared a high level of similarity to a Salmonella-specific chromosomal sequence. Examination of other known IS911-like sequences showed that these sequences also were frequently associated with other accessory elements and appeared to be prone to partial deletion events. Analysis of the data led to a model of the evolution of this unusual composite locus.

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