AN INSERTION ELEMENT OF THE EXTREMELY THERMOPHILIC ARCHAEON SULFOLOBUS-SOLFATARICUS TRANSPOSES INTO THE ENDOGENOUS BETA-GALACTOSIDASE GENE

Three phenotypically stable mutants of the extremely thermophilic arch aeon Sulfolobus solfataricus have been isolated by screening for beta- galactosidase negative colonies on plates with X-Gal romo-4-chloro-3-i ndolyl-beta-D-galactopyranoside). From one of these mutants an inserti on element, designated ISC1217, was isolated and characterized. Sequen ce analysis of ISC1217 and of the regions adjacent to the insertion si te in the beta-galactosidase gene revealed features typical of a trans posable element: ISC1217 contained terminal inverted repeats and was f lanked by a direct repeat of 6 bp. The 1147 bp sequence contained an o pen reading frame encoding a putative protein of 354 amino acid residu es and, overlapping this, two smaller open reading frames on the oppos ite strand. There were approximately 8 copies of the insertion element in the S. solfataricus genome. ISC1217 did not cross-hybridize with D NA of other Sulfolobus species. All three independently isolated beta- galactosidase mutants of S. solfataricus arose by transposition of ISC 1217 or a related element.