ISBst12, a novel type of insertion-sequence element causing loss of S-layer-gene expression in Bacillus stearothermophilus ATCC 12980

摘要

The cell surface of the surface layer (S-layer)-carrying strain of Bacillus stearothermophilus ATCC 12980 is completely covered with an oblique lattice composed of the S-layer protein SbsC. In the S-layer-deficient strain, theS-layer gene sbsC was still present but was interrupted by a novel type of insertion sequence (IS) element designated ISBst12. The insertion site was found to be located within the coding region of the sbsC gene, 199?bp downstream from the translation start of SbsC. ISBst12 is 1612?bp long, bounded by 16?bp imperfect inverted repeats and flanked by a directly repeated 8?bp target sequence. ISBst12 contains an ORF of 1446?bp and is predicted to encode a putative transposase of 482?aa with a calculated theoretical molecular mass of 55562?Da and an isoelectric point of 9·13. The putative transposase does not exhibit a typical DDE motif but displays aHis-Arg-Tyr triad characteristic of the active site of integrases from the bacteriophage λ Int family. Furthermore, two overlapping leucine-zipper motifs were identified at the N-terminal part of the putative transposase. As revealed by Southern blotting, ISBst12 was present in multiple copies in the S-layer-deficient strain as well as in the S-layer-carrying strain. Northern blotting indicated that S-layer gene expression is already inhibited at the transcriptional level, since no sbsC-specific transcript could be identified in the S-layer-deficient strain. By using PCR, ISBst12 was also detected in B. stearothermophilus PV72/p6, in its oxygen-induced strain variant PV72/p2 and in the S-layer-deficient strain PV72/T5.