High-frequency genomic rearrangements involving archaebacterial repeat sequence elements

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Abstract
Halobacterium halobium is an obligately halophilic archaebacterium1 of interest to molecular biologists for many reasons2–5, one of which is the unexplained high frequency (10−4–10−2 mutants per cell plated) at which it yields readily identifiable and unstable mutants4. We showed previously5 that the genome of H. halobium contains many (>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 (>4 × 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.