High-frequency spontaneous mutation in the bacterio-opsin gene in Halobacterium halobium is mediated by transposable elements.

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Abstract
A transposable element, ISH1, which inactivates the bacterio-opsin (BO) gene in 2 purple membrane-deficient (Pum-) mutants of H. halobium. Examination of 9 additional Pum- mutants now shows that in all of these the BO gene was inactivated by insertion of 1 of 2 types of transposable elements. Four Pum- strains contain ISH1 within the BO gene, probably at the same site that was previously characterized. A second element, ISH2, which is present in 4 more strains, inserts at multiple sites within the BO coding sequence. Another Pum- strain contains the ISH2 element 102 nucleotides upstream from the iniator codon for BO. ISH2, which is 520 nucleotides long, is the smallest insertion sequence known. Its sequence was determined; it is A + T rich (53%), contains a 19-base-pair inverted repeat at its terminal and duplicates 10 or 20 base pairs at the target site during insertion. ISH2 is present in multiple copy numbers in the genome and contains several relatively short open reading frames.