All roads lead to arginine: the squid protamine gene.

The protamine of squid is one of the most arginine-rich protamines (77%, mol/mol). It possesses a leading sequence that is posttranslationally removed during spermatogenesis in a manner that is analogous to that observed in some of its vertebrate protamine counterparts. In this paper we describe the gene sequence of the protamine of the squid Loligo opalescens. This represents the first complete gene sequence ever reported for an invertebrate protamine. Like those of vertebrate protamines, the messenger RNA is polyadenylated but the gene does not contain an intron. The promoter region contains the major transcriptional regulatory elements (CRE, TATA box, and CAP) that are also characteristic of the vertebrate protamine genes. It is unclear whether the similarities of protamines in species from both the deuterostome and the protostome branches represent the result of phylogenetic conservation or evolutionary convergence.