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, coinc
ident with the radiation-induced G(2) block. In order to understand th
e mechanisms underlying these variations, we have measured cyclin B1 m
RNA 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 mod
ulating 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.