The human somatic cytochrome c gene: two classes of processed pseudogenes demarcate a period of rapid molecular evolution.

We have isolated and determined the DNA sequences of the human somatic cytochrome c gene (HCS) and 11 processed pseudogenes. HCS is the functional homologue to the previously characterized rat somatic gene because it correctly encodes the human heart protein, is present in single copy in the human genome, is nearly identical in both size and intron/exon structure to rodent somatic genes, and shares a high degree of sequence homology with its rat counterpart including a well-conserved promoter region (77% over 250 nucleotides). In contrast to the rodent system, however, where the known pseudogenes all originated from a locus encoding the present day cytochrome c, the human pseudogenes are of two types. A predominant class of older pseudogenes came from a progenitor of HCS that encoded an ancestral form of the protein, while a second group of only a few young pseudogenes originated from a recent parent of HCS that encoded the current cytochrome c polypeptide. These two distinct classes of human pseudogenes provide a molecular record of the history of cytochrome c evolution in primates and demarcate a short period of rapid evolution of the functional gene.