THE STEADY-STATE LEVELS AND STRUCTURE OF THE U7 SNRNP ARE CONSTANT DURING THE HUMAN CELL-CYCLE - LACK OF CELL-CYCLE REGULATION OF HISTONE MESSENGER-RNA 3' END FORMATION

The U7 small nuclear ribonucleoprotein (snRNP) is an essential compone nt of the endonucleolytic cleavage reaction which leads to the product ion of mature 3'-ends of histone premRNAs. We have examined the relati ve amount and the structure of the U7 snRNP, as assayed by sensitivity to micrococcal nuclease, during the cell cycle in human HeLa and WI-3 8 cells. Using an RNase A protection assay, we find no change in the s teady state levels of U7 throughout the cell cycle. Similarly, the sen sitivity of U7 to micrococcal nuclease remained unchanged in both cell types. Contact inhibited WI-38 cells, that are deemed to have left th e cell cycle and entered a quiescent state, displayed similar levels o f U7 to cells in S and G1 phases of the cell cycle, however, the U7 sn RNA was slightly more resistant to micrococcal nuclease. Histone 3' en d mRNA processing was also assayed in HeLa cell cycle phase-specific e xtracts. In marked contrast to previous observations in extracts prepa red from the rodent cell line, C3H10T1/2, (Hoffmann and Birnstiel, 199 0), we find that the 3' end processing reaction remained constant thro ughout the cell cycle.