High-frequency genomic rearrangements involving archaebacterial repeat sequence elements.

Halobacterium halobium is an obligately halophilic archaebacterium of interest to molecular biologists for many reasons, one of which is the unexplained high frequency (10(-4)-10(-2) mutants per cell plated) at which it yields readily identifiable and unstable mutants. We showed previously that the genome of H. halobium contains many (greater than 50) families of repeated sequences whose members are dispersed on both chromosome and plasmid. Here we report that most if not all of the members of most of these repeat sequence families effect or are affected by spontaneous genomic rearrangements. Quantitative analyses show that such repeat sequence-associated rearrangements (which may be of several kinds) occur at high frequencies (greater than 4 x 10(-3) events per family per cell generation), while unique-sequence DNAs are physically stable. The presence of so many families of elements of such great instability in the halobacterial genome gives it an unusual degree of structural and perhaps functional plasticity.