NESTED DNA INVERSION AS A PARADIGM OF PROGRAMMED GENE REARRANGEMENT

Programmed gene rearrangements are employed by a variety of microorgan isms, including viruses, prokaryotes, and simple eukaryotes, to contro l gene expression. In most instances in which organisms mediate host e vasion by large families of homologous gene cassettes, the mechanism o f variation is not thought to involve DNA inversion. Here we report th at Campylobacter fetus, a pathogenic Gram-negative bacterium, reassert s a single promoter, controlling surface-layer protein expression, and one or more complete ORFs strictly by DNA inversion. Rearrangements w ere independent of the distance between sites of inversion. These rear rangements permit variation in protein expression from the large surfa ce-layer protein gene family and suggest an expanding paradigm of prog rammed DNA rearrangements among microorganisms.