A novel group-II intron in the cox1 gene of the fission yeast Schizosaccharomyces pombe is inserted in the same codon as the mobile group-II intron al2 in the Saccharomyces cerevisiae cox1 homologue

We describe herein a large group-II intron which is inserted in the mitocho ndrial cox1 gene of the Schizosaccharomyces pombe strain EF2. The intron RN A consists of 2492 nucleotides which can be folded into a secondary structu re with all the expected sequence motifs of subgroup-IIA 1 introns (Michel et al. 1989). Determination of the exact splice point revealed that the int ron is inserted in the same codon, but 1 bp downstream, as the mobile intro n aI2 in the Saccharomyces cerevisiae cox1 homologue. A total of nine nucle otide changes was observed around the insertion site of the intron in the c ox1 gene of strain EF2 compared with the reference strain ade7-50h(-). Seve n of these changes are clustered within the 51 bp upstream of the splice po int. Only one sequence deviation was found in the downstream exon. The intr on is capable of splicing splicing despite the fact that both the EBS1/IBS1 and the EBS2/IBS2 sequence motifs, thought: to be necessary for correct sp licing, extend over 5 instead of 6 bp. The maturase, endonuclease and rever se transcriptase domains of the putative protein encoded by the newly descr ibed S. pombe group-II intron were not closer to those encoded by the other two, cobI and cox2I, S. pombe group-II introns than to the group-II intron -encoded proteins in Allomyces Marchantia, Podospora and Saccharomyces.