The presequence of cytochrome c1 from potato mitochondria is encoded on four exons

The structural organization of a nuclear gene encoding cytochrome c₁ from potato was determined. The gene spans 5.1 kb and contains eight introns. All intron/exon junctions follow the GT/AG rule. Functional domains of the mature cytochrome c₁ protein are located on separate exons. The presequence, which targets the cytochrome c₁ precursor to the mitochondrion and to the correct intra-mitochondrial location, is encoded on the first four exons. The largest intron (2.8 kb) separates the information for mitochondrial targeting from the intra-mitochondrial sorting domain of the cytochrome c₁ protein. In contrast to other organellar precursor proteins, there is no intron between the DNA sequence encoding the presequence and the mature protein. This may indicate that during evolution the genetic information for the prokaryotic cytochrome c₁ was transferred to the nucleus together with the bacterial secretion signal which is structurally and functionally related to intramitochondrial sorting domains.