The mosaic organization of the apocytochrome b gene of Aspergillus nidulans revealed by DNA sequencing.

The coding section of the apocytochrome b gene (cobA) of the mitochondrial DNA of Aspergillus nidulans has been completely sequenced. The gene comprises two exons of 507 and 654 bp separated by one intron of approximately 1.1 kb. The derived amino acid sequence shows 61% homology with that of Saccharomyces cerevisiae and 51% homology with the cognate human sequence. Comparison of these sequences indicates that UGA codes for tryptophan in the A. nidulans mitochondrial system. The intron is in exactly the same place as intron 13 of the apocytochrome b gene of "long" strains of S. cerevisiae. Other introns present in "long" strains (I1 and I2) are absent in A. nidulans. Introns 14 and 15 present both "long" and "short" strains of yeast are also absent. This is particularly significant in view of the involvement of 14 in the splicing of both cob and oxi3 (coding for cytochrome oxidase subunit 1) mRNA precursors in S. cerevisiae. The partial sequence of the A. nidulans intron shows an open reading frame for at least the first 200 bp, and shows the intron to be closed in all frames at the carboxyl terminus.