The Biosynthesis of Protamine in Trout Testis

Protamines are synthesized in the cytoplasm of rainbow trout (Salmo gairdnerii) spermatid cells. Following a pulse of 14C-arginine, the majority of the labeled nascent protamine is found on diribosomes (disomes) sedimenting at 120 S (Ling, V., Trevithick, J. R., and Dixon, G. H., Can. J. Biochem., 47, 51 (1969)). Polysome profiles obtained at various times during testis maturation, which, in this species, takes place from September to December, show the appearance of high concentrations of such disomes during the period of rapid protamine synthesis. This effect is not observed in trout liver polysomes isolated under the same conditions, the polysome profiles being independent of the sexual maturation of the fish. Spermatid cell disomes are not disaggregated by lowering Mg++ concentration during isolation or by the addition of pancreatic ribonuclease. Pulse-chase experiments with 14C-arginine show that disomes are labeled more rapidly than heavier polysomes. The distribution of incorporated 14C-arginine and 14C-lysine through the polysome profiles changes markedly during the maturation of the testis; at the time of rapid protamine synthesis, the disomes incorporate arginine rapidly but lysine slowly, suggesting that they are largely engaged in protamine synthesis. Actinomycin D has an inhibitory effect on the incorporation of 14C-arginine into protamine only during the early protamine stage of development, suggesting that messenger RNA for protamine is synthesized before protamine synthesis takes place and is metabolically stable.