The fate of cytoplasmic histone mRNA was studied under conditions in which initiation of protein synthesis in synchronized HeLa cells is S phase was blocked by increasing the osmolarity of the growth medium with NaCl. In contrast to the interruption of DNA replication with hydroxyurea, which results in an exponential degradation of translatable histone mRNA with a half-life of about 10-13 min, blocking the initiation of protein synthesis leads to only a marginal loss of biologically active histone mRNA in the cytoplasm. When the initiation of protein synthesis was interrupted by treating cells with 150 mM NaCl, 40-50% of the total cytoplasmic histone mRNA previously translated in polyribosomes appears in the cytoplasm integrated into mRNA-protein particle(s) sedimenting between 15 S and 30 S. On the other hand, in untreated S-phase cells or in cells blocked with hydroxyurea only 3-6% of the total translatable histone mRNA is found in the cytoplasm not bound to ribosomes or their subunits. In addition, the degradation of histone mRNA in hydroxyurea-blocked S-phase cells is prevented when the initiation of protein synthesis is inhibited with NaCl. These studies clearly indicate that the inhibition of initiation of protein synthesis per se is not the cause for the rapid degradation of cytoplasmic histone mRNA observed when DNA replication is turned off and that the inactivation of these mRNAs is a process dependent on continuous protein synthesis.
Cell Division Centrifugation, Density Gradient