RNA Metabolism in Primary Cultures of <I>Xenopus laevis</I> Kidney Cells. I. Characterization of Growth Phases and Changes in Number and Size of Nucleoli

RNA Metabolism in Primary Cultures of <I>Xenopus laevis</I> Kidney Cells. II. Comparison of the Pattern of RNA Synthesis at Different Phases of Growth

“…The amount of the label incorporation into total RNA was relatively high at the lag phase, increased slightly at the logarithmic phase and decreased greatly at the stationary phase. These observations are consistent with our previous results (8). The labeling pattern was quite similar between the lag and logarithmic phases.…”

“…The present paper compared the pattern of RNA synthesis in the three different phases of growth, and it was found that the relative labeling of 40S ribosomal RNA precursor increased greatly when cellular proliferation was reduced at the stationary phase. As has been suggested in our previous paper (8) and is generally known in other cultured cell systems (3,4,10), the transcription of 40S ribosomal RNA precursor may be repressed rather than being activated when cells become contact-inhibited at the stationary phase of growth. Furthermore, it is highly unlikely that the relative instability of 18S and 28S ribosomal RNA in stationary phase cells (1, 2, 5, 10) could bring about a significant lowering of the amount of labeled 18S and 28S ribosomal RNA, since it is known that mature ribosomal RNA is quite stable at least shortly after labeling.…”

“…In our previous paper, we followed 31-1-uridine uptake and incorporation during the growth cycle of Xenopus kidney cells and obtained results which suggested that cellular RNA synthesis became considerably inactivated at the stationary phase (8). The present paper compared the pattern of RNA synthesis in the three different phases of growth, and it was found that the relative labeling of 40S ribosomal RNA precursor increased greatly when cellular proliferation was reduced at the stationary phase.…”

“…However, the mechanism by which such regulation is realized is still unclear (1,2,3,4,5,10). In the present paper, the pattern of RNA synthesis was compared at different growth phases of primary cultures of Xenopus laevis kidney cells (8). It was observed that the proportion of labeling of 40S ribosomal RNA precursor to that of 18S and 28S mature ribosomal RNA increased remarkably when the logarithmic phase cells entered the stationary phase.…”