Synthesis of RNA by Xenopus spermatogenic cells. II. ATP pool kinetics, relative rates of RNA accumulation and improved separation of cell populations.

Abstract

Rates of accumulation of newly synthesized RNA molecules were compared in individual spermatogenic cell stages of Xen opus laevis maintained in vitro. Direct comparisons were made possible by using [3H1-adenosine as an RNA precursor in appropriate labeling protocols in conjunction with determination of specific activity of adenosine triphosphate (ATP) pools. It was determined that with short (1 h) labeling periods, spermatogonia were the most active cells with respect to RNA synthesis. Meiotic prophase cells showed a substantial, but significantly diminished, rate of RNA synthesis. RNA synthesis in postmeiotic haploid stages continued to decline, with spermatozoa exhibiting minimal synthetic activity. After equilibration of radioactivity in precursor pools with longer (3-12 h) periods of incorporation, significant decreases compared with a 1 h labeling period were found in the net accumulation of label into RNA in individual cell stages. Nevertheless, the same relative order of net accumulation persisted. Decreased rates of accumulation of newly synthesized RNA in spermatids relative to meiotic prophase cells were greater than could be accounted for by simple differences in ploidy. It is concluded that 1) net rates of accumulation and turnover of RNA are significantly different during different stages of spermatogenesis; 2) transcription of the genome as a whole is progressively restricted throughout spermatogenesis; and 3) rates of RNA accumulation are not simply proportional to the DNA content of individual stages.