Kumar A
J Cell Biol. 1970 Jun;45(3):623-34. doi: 10.1083/jcb.45.3.623.
The cellular site of synthesis of ribosomal RNA in Tetrahymena pyriformis was studied by analyzing the purified nuclear and cytoplasmic RNA from cells pulse labeled with uridine-(3)H. The results of studies using zonal centrifugation in sucrose density gradients show that the ribosomal RNA is synthesized in the nucleus as a large precursor molecule sedimenting at 35S. The 35S molecule undergoes rapid transformation through two main nuclear intermediates, sedimenting at about 30S and 26S. The smaller ribosomal RNA (17S) appears first in the cytoplasm and it seems to be absent from the nucleus. The apparent delay in the appearance of the larger ribosomal RNA (26S) in the cytoplasm is due to the presence of a larger pool of its precursors in the nucleus as indicated by pulse-chase experiments. The newly synthesized ribosomal RNA's appear in the cytoplasm as discrete 60S and 45S ribonucleoprotein particles, before their incorporation into the polysomes.
通过分析用尿苷 -(3)H脉冲标记的细胞中纯化的核RNA和细胞质RNA,研究了梨形四膜虫核糖体RNA的细胞合成位点。在蔗糖密度梯度中进行区带离心的研究结果表明,核糖体RNA在细胞核中作为沉降系数为35S的大前体分子合成。35S分子通过两个主要的核中间体快速转变,沉降系数约为30S和26S。较小的核糖体RNA(17S)首先出现在细胞质中,而在细胞核中似乎不存在。脉冲追踪实验表明,较大的核糖体RNA(26S)在细胞质中出现明显延迟是由于其前体在细胞核中的含量较高。新合成的核糖体RNA在掺入多核糖体之前,以离散的60S和45S核糖核蛋白颗粒的形式出现在细胞质中。