Sequence divergence of rainbow trout protamine mRNAs; comparison of coding and non-coding nucleotide sequences in three protamine cDNA plasmids

Abstract

SPERMATOGENESIS in the rainbow trout (Salmo gairdnerii) is associated with the Onset of synthesis of the protamines^1,2, a family of small, highly basic proteins consisting of 32 or 33 residues, of which approximately two-thirds are arginine³. By displacing the histones, the protamines have a unique role in packaging sperm DNA. The biosynthesis of the protamines has been studied in some detail, and polyadenylated protamine mRNA of about 290 nucleotides has been isolated from trout testis^4,5 and shown by R₀t analysis to be made up of three to four components^5,6. More recent work suggests that there are about six different protamine genes per haploid genome⁷. In contrast to the interspecific conservation of histone sequences, the observed rate of protamine sequence divergence between the closely related clupeid fishes such as trout, salmon and herring, is very high, and there is evidence that protamine genes have arisen following a doubling of the ancestral fish genome. Comparison of the cloned complementary DNA sequences and ultimately of the genes themselves will help to elucidate the molecular events involved in the evolution of these proteins. To study the chromosomal organisation of these genes and the control of their expression during spermatogenesis, I have constructed cDNA clones using purified poly(A)⁺ mRNA as starting material. Here, the sequences of three of these recombinants are compared, revealing an unexpected pattern of divergence in the coding and non-coding regions, as well as a mutational ‘hot-spot’ corresponding to the major phosphorylation site of the protamines.