EVOLUTION OF PROTAMINE P1 GENES IN PRIMATES

Protamine P1 genes have been sequenced by PCR amplification and direct DNA sequencing from 9 primates representing 5 major families, Cebidae (new world monkeys), Cercopithecidae (old world monkeys), Hylobatidae (gibbons), Pongidae (gorilla, orangutan, and chimpanzee), and Hominid ae (human). In this recently diverged group of primates these genes ar e clearly orthologous but very variable, both at the DNA level and in their expressed amino acid sequences. The rate of variation amongst th e protamine P1s indicates that they are amongst the most rapidly diver ging polypeptides studied. However, some regions are conserved both in primates and generally in other placental mammals. These are the 13 N -terminal residues (including a region of alternating serine and argin ine residues (the motif SRSR, res. 10-13) susceptible to Ser phosphory lation), a tract of six Arg residues (res. 24-29) in the center of the molecule, and a six-residue region (RCCRRR, res. 39-44), consisting o f a pair of cysteines flanked by arginines. Detailed consideration of nearest-neighbor matrices and trees based on maximum parsimony indicat es that PI genes from humans, gorillas, and chimpanzees are very simil ar. The amino acid and nucleotide differences between humans and goril las are fewer than those between humans and chimpanzees. This finding is at variance with data from DNA-DNA hybridization and extensive glob in and mitochondrial DNA sequences which place human and chimpanzee as closest relatives in the super family, Hominoidea. This may be relate d to the fact that protamine P1s are expressed in germ line rather tha n somatic cells. In contrast to the variability of the exon regions of the protamine P1 genes, the sequence of the single intron is highly c onserved.