Cloning of bovine P1 protamine cDNA and the evolution of vertebrate P1 protamines

A bovine P1 protamine cDNA from a bull testis cDNA library was isolated utilizing a series of oligonucleotide probes. Sequence analysis showed that the cloned cDNA insert extended 317 bp to the poly(A) tail. The 51-residue 6750-dalton protamine primary translated protein is encoded within a 156-bp segment. The protamine sequence predicted from the cDNA sequence differs from that previously reported for the amino acid sequence of bovine protamine P1 by the insertion of the tripeptide Cys-Arg-Arg from residues 39-41 in the carboxy-terminal region of the mature protein. Consistent with previous hybridization analysis, nucleotide sequence comparisons showed that trout protamine cDNA was more closely related to that of bovine than to that of mouse. However, bovine P1 protamine cDNA shared greater sequence homology with mouse P1. A common nucleotide sequence of 30 bp is conserved among all three of these species. Primer extension analysis revealed that, as with trout protamine mRNAs, the majority of the untranslated portion of the mRNA lies 3' to the coding segment. Comparisons of their mRNA secondary structures by computer modeling indicate that the mRNAs fold back onto themselves, producing similar, extensively hydrogen-bonded, convoluted forms. These models support the view that translational regulation of protamine mRNA may be partially dependent on secondary structure. Southern analysis suggests that the bovine protamine P1 gene is not sex-linked and is present as one (or relatively few) copy within the bovine genome.