EVIDENCE FOR POSTTRANSCRIPTIONAL REGULATION OF CYCLIN B1 MESSENGER-RNA IN THE CELL-CYCLE AND FOLLOWING IRRADIATION IN HELA-CELLS

Cyclin B1 mRNA expression varies markedly through the cell cycle with its peak in G(2)/M and lowest level in G(1). Cyclin B1 mRNA levels are also transiently reduced in HeLa cells after gamma-irradiation, coincident with the radiation-induced G(2) block. In order to understand the mechanisms underlying these variations, we have measured cyclin B1 mRNA stability in HeLa cells during different phases of the cell cycle. The half-life of the mRNA measured after actinomycin D administration is 1.1-1.8 h in both early and late G(1), 8 h in S and 13 h in G(2)/M. We therefore conclude that altered RNA stability is important in modulating cyclin B1 mRNA levels through the HeLa cell cycle. Furthermore, 3 h after irradiation of HeLa cells in S phase with 10 Gy, the half-life of cyclin B1 mRNA is reduced to 5 h; it is further reduced to 2-3 h at 14 h after irradiation. Thus, decreased stability contributes to the reduction in cyclin B1 mRNA following irradiation.