An unusual form of purifying selection in a sperm protein

Protamines are small, highly basic DNA-binding proteins found in the sperm of animals. Interestingly, the proportion of arginine residues in one type of protamine, protamine P1, is about 50% in mammals. Upon closer examinatio n, it was found that both the total number of amino acids and the positions of arginine residues have changed considerably during the course of mammal ian evolution. This evolutionary pattern suggests that protamine P1 is unde r an unusual form of purifying selection, in which the high proportion of a rginine residues is maintained but the positions may vary. In this case, we would expect that the rate of nonsynonymous substitution is not particular ly low compared with that of synonymous substitution, despite purifying sel ection. We would also expect that the selection for a high arginine content results in a high frequency of the nucleotide G in the coding region of th is gene, because all six arginine codons contain at least one G. These expe ctations were confirmed in our study of mammalian protamine genes. Analysis of nonmammalian vertebrate genes also showed essentially the same patterns of evolutionary changes, suggesting that this unusual form of purifying se lection has been active since the origin of bony vertebrates. The protamine gene of an insect species shows similar patterns, although its purifying s election is less intense. These observations suggest that arginine-rich sel ection is a general feature of protamine evolution. The driving force for a rginine-rich selection appears to be the DNA-binding function of protamine P1 and an interaction with a protein kinase in the fertilized egg.