Archaeal pre-mRNA splicing: a connection to hetero-oligomeric splicing endonuclease.

Eukaryotic Cbf5 is a protein subunit of the small nucleolar RNA-protein complex. Previously, we identified, in archaeal homologs of cbf5 of the crenarchaea, Aeropyrum pernix, Sulfolobus solfataricus, and Sulfolobus tokodaii, the first examples of introns of archaeal protein-coding genes. Here, we report the immunological detection of Cbf5 protein of S. tokodaii, the product of the spliced cbf5 mRNA. The hetero-oligomeric splicing endonuclease activity from recombinant S. tokodaii subunits cleaved at the exon-intron boundaries of cbf5 pre-mRNA fragments,suggesting that synthesis of full-length Cbf5 protein requires this activity. Database searches and PCR screens identified additional cbf5 introns in some, but not all sequenced crenarchaeal genomes. The predicted secondary structures of exon-intron boundaries of many of the newly identified intron-containing cbf5 pre-mRNAs contained relaxed forms of the bulge-helix-bulge motif similar to that of S. tokodaii. These observations are consistent with previous reports indicating that subunit composition of the splicing endonuclease contributes to substrate specificity.