Internal structure of a mitochondrial intron of Aspergillus nidulans.

The intron of the mitochondrial apocytochrome b gene, cobA, of Aspergillus nidulans has been subjected to sequence analysis. It contains an open reading frame of 957 base pairs contiguous with the preceding exon. Regions of the translated open reading frames of cobA and the third intron of the cob gene in yeast show high amino acid homology. Comparison of the cobA intron with this and other yeast introns indicates that cobA codes for a maturase protein that splices out the intron encoding it and possibly other mitochondrial introns. Two very similar decamer peptides are found in the protein sequences of the cobA intron, four mitochondrial yeast introns, and the yeast mitochondrial sequence reading frame 1 (RF-1) and may be diagnostic of one class of maturase-coding introns. Four short DNA sequences, two of which are in the region defined by box9 and box2 mutations in the cob gene of yeast, are conserved in cobA and certain yeast introns. Comparison with three yeast introns strongly suggests that the first 200 base pairs of the open reading frame of the cobA intron do not code for any amino acids present in the putative maturase protein but are required for splicing or the control of splicing, or both.