Bond U, Yario T A
Biology Department, Fairfield University, CT 06430.
Cell Mol Biol Res. 1994;40(1):27-34.
The U7 small nuclear ribonucleoprotein (snRNP) is an essential component of the endonucleolytic cleavage reaction which leads to the production of mature 3'-ends of histone premRNAs. We have examined the relative amount and the structure of the U7 snRNP, as assayed by sensitivity to micrococcal nuclease, during the cell cycle in human HeLa and WI-38 cells. Using an RNase A protection assay, we find no change in the steady state levels of U7 throughout the cell cycle. Similarly, the sensitivity of U7 to micrococcal nuclease remained unchanged in both cell types. Contact inhibited WI-38 cells, that are deemed to have left the cell cycle and entered a quiescent state, displayed similar levels of U7 to cells in S and G1 phases of the cell cycle, however, the U7 snRNA was slightly more resistant to micrococcal nuclease. Histone 3' end mRNA processing was also assayed in HeLa cell cycle phase-specific extracts. In marked contrast to previous observations in extracts prepared from the rodent cell line, C3H10T1/2, (Hoffmann and Birnstiel, 1990), we find that the 3' end processing reaction remained constant throughout the cell cycle.
U7小核核糖核蛋白（snRNP）是内切核酸酶切割反应的重要组成部分，该反应导致组蛋白前体mRNA成熟3'端的产生。我们通过微球菌核酸酶敏感性分析，研究了人HeLa和WI-38细胞在细胞周期中U7 snRNP的相对含量和结构。使用核糖核酸酶A保护分析，我们发现在整个细胞周期中U7的稳态水平没有变化。同样，在两种细胞类型中，U7对微球菌核酸酶的敏感性保持不变。接触抑制的WI-38细胞被认为已离开细胞周期并进入静止状态，其U7水平与细胞周期S期和G1期的细胞相似，然而，U7 snRNA对微球菌核酸酶的抗性略强。还在HeLa细胞周期阶段特异性提取物中检测了组蛋白3'端mRNA加工。与之前在啮齿动物细胞系C3H10T1/2制备的提取物中的观察结果（霍夫曼和伯恩斯蒂尔，1990）形成鲜明对比的是，我们发现在整个细胞周期中3'端加工反应保持恒定。
