RNA synthesis and RNA polymerase activities in germ cells of developing rainbow trout testis

Spermatogenesis is a complex developmental process which sequentially generates several different germ cell types. These cell types from rainbow trout (Salmo gairdnerii) testis were separated by sedimentation in serum albumin gradients and characterized on the basis of their physical properties, chronological appearance, and protein synthesis. The rate of RNA synthesis, the types of RNA made, and the RNA polymerase activities present were determined for each cell type. The rate of RNA synthesis decreased from a high level in spermatogonia and spermatocytes to a low level in early spermatids and was absent in late spermatids and mature spermatozoa. Newly synthesized RNA in spermatogonia and spermatocytes consisted of a variety of molecular weight species, including 18 S and 28 S ribosomal RNAs. The synthesis of high molecular weight RNAs, especially ribosomal RNAs, decreased drastically in early spermatids, leading to the synthesis of only small molecular weight RNAs. RNA polymerase I and II were present in all cell types but the activities of both showed large decreases between spermatocytes and middle spermatids. Both RNA polymerase activities were almost absent from spermatozoa. The activities of RNA polymerase I and II from unfractionated testis cells at different stages of hormone-induced spermatogenesis were quantitated by fractionation of the solubilized extract on DEAE-cellulose. Both polymerases showed major decreases in activity which began near the chronological mid-point of development. For polymerase I the decrease in activity was over 400 fold, for polymerase II over 200 fold. The number of RNA polymerase II molecules per testis cell, quantitated by the binding of [3H]amanitin to cell extracts, also decreased markedly during spermatogenesis. The reduction in polymerase II activity was accompanied by a parallel 200-fold decrease in[3H]amanitin binding. The reduction in polymerase activity appears, therefore, to be due to an actual reduction in the cellular content of RNA polymerase II molecules. These results suggest that transcription in maturing testes is regulated, at least in part, by the concentrations of the RNA polymerases.