Cytochrome b gene of Neurospora crassa mitochondria. Partial sequence and location of introns at sites different from those in Saccharomyces cerevisiae and Aspergillus nidulans.

We have sequenced a 2614-base pair fragment of the Neurospora crassa mitochondrial DNA which contains part of the structural gene for apocytochrome b. This gene is split by at least two introns. The sequence reported here begins within one intron, extends through the next exon, another intron 1276 base pairs long, and the last exon which encodes the COOH terminus of cytochrome b. Within the 254 amino acids encoded by the two exons, there is a high degree of sequence conservation, 81%, with cytochrome b of Aspergillus nidulans. Surprisingly, both introns in the N. crassa cytochrome b gene are located at positions different from introns in the corresponding genes in Saccharomyces cerevisiae or A. nidulans. The upstream intron is located 22 nucleotides before the first intron in the long form of the S. cerevisiae cytochrome b gene. The downstream intron is located 16 nucleotides before the third intron in the long form of the S. cerevisiae gene and the only intron in the A. nidulans cytochrome b gene. The 1276-base pair downstream intron contains a 314 amino acid long open reading frame, which is in-phase with the preceding exon. The protein product of this reading frame has some resemblance to intron-encoded proteins, known as "mRNA maturases," which are thought to participate in RNA splicing in the mitochondria of S. cerevisiae. Another feature shared by the downstream intron and most other mitochondrial introns is the presence of the Box 9 and Box 2 consensus sequences, which may also be important for RNA splicing.