Cytochrome c gene-related sequences in mammalian genomes

We use a rat cytochrome c gene that we previously isolated and determined the sequence of to estimate the number of related sequences present in the rat genome. Approximately 25 different EcoRI restriction endonuclease fragments from total rat DNA hybridize to the gene of known structure. Four of these correspond to homologous sequences present in four different lambda Charon 4A-rat cytochrome c recombinants previously isolated. Intact or nearly intact genes appear to reside on almost all of the genomic fragments, because they hybridize strongly to gene subfragments representing both 5' and 3' portions of the coding sequence as well as to 3' noncoding DNA that is found specifically associated with the coding region. A subgroup of about six of the fragments also shares homology within the 73 nucleotides immediately preceding the AUG codon. An intron-specific probe reveals only the EcoRI fragment from which it was derived and one other genomic fragment. On the basis of the temperature of complete dissociation of the coding region probe in 0.75 M NaCl/0.075 M Na3 citrate/50% (vol/vol) formamide, the 25 fragments are separable into three stringency classes of 40-50 degrees C, 50-55 degrees C, and 55-60 degrees C. The latter, high-stringency group of about seven fragments includes those cloned in the recombinant phage isolates, whose regions homologous to cytochrome c are shown to differ from the purified gene of known sequence by an amount equivalent to about 2% mismatched bases. Families of cytochrome c gene-related sequences are also found in the genomes of several other mammals, including humans.