CONSERVATION OF CHROMOSOMAL ARRANGEMENT AMONG 3 STRAINS OF THE GENETICALLY UNSTABLE ARCHAEON HALOBACTERIUM-SALINARIUM

Phenotypic variants of Halobacterium salinarium NRC-1 arise at a frequ ency of 10(-2). These result from transpositions of halobacterial inse rtion sequences and rearrangements mediated by halobacterial insertion sequences. We have tested the hypothesis that such mutations are conf ined to only a portion of the genome by comparing the chromosomal rest riction map of H. salinarium NRC-1 and that of the derivative S9, whic h was made in 1969. The two chromosomes were mapped by using two-dimen sional pulsed-held gel electrophoresis and the restriction enzymes Afl II, AseI, and DraI. A comparison of the two deduced maps showed a doma in of about 210 kbp to be subject to many rearrangements, including an inversion in So relative to NRC-1. However, the rest of the chromosom e was conserved among NRC-I, So, and an independent Halobacterium isol ate, GRB, previously mapped by St. Jean et al. (A. St. Jean, B. A, Tri eselmann, and R. L. Charlebois, Nucleic Acids Res. 22:1476-1483, 1994) . This concurs with data from eubacteria suggesting strong selective f orces maintaining gene order even in the face of rearrangement events occurring at a high frequency.