Group II intron in Bacillus cereus has an unusual 3' extension and splices 56 nucleotides downstream of the predicted site.
All group II introns known to date fold into six functional domains. However, we recently identified an intron in Bacillus cereus ATCC 10987, B.c.I4, that splices 56 nt downstream of the expected 3' splice site in vivo (Tourasse et al. 2005, J. Bacteriol., 187, 5437-5451). In this study, we confirmed by ribonuclease protection assay that the 56-bp segment is part of the intron RNA molecule, and computational prediction suggests that it might form a stable stem-loop structure downstream of domain VI. The splicing of B.c.I4 was further investigated both in vivo and in vitro. Lariat formation proceeded primarily by branching at the ordinary bulged adenosine in domain VI without affecting the fidelity of splicing. In addition, the splicing efficiency of the wild-type intron was better than that of a mutant construct deleted of the 56-bp 3' extension. These results indicate that the intron has apparently adapted to the extra segment, possibly through conformational adjustments. The extraordinary group II intron B.c.I4 harboring an unprecedented extra 3' segment constitutes a dramatic example of the flexibility and adaptability of group II introns.
Stabell
Fredrik B
FB
Department of Pharmaceutical Biosciences, University of Oslo, Oslo, Norway.
Tourasse
Nicolas J
NJ
Ravnum
Solveig
S
Kolstø
Anne-Brit
AB
D001409:Bacillus cereus / Q000235:genetics*; D001483:Base Sequence; D007438:Introns; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D012313:RNA / Q000737:chemistry; D022821:RNA Splice Sites; D012326:RNA Splicing; D012329:RNA, Bacterial / Q000737:chemistry* / Q000378:metabolism; D000079962:RNA, Circular; D018626:Retroelements
D022821:RNA Splice Sites; D012329:RNA, Bacterial; D000079962:RNA, Circular; D018626:Retroelements; D012313:RNA