Our objective is to elucidate the factors involved in the close co- ordination of the synthesis of histone protein and DNA. Recently we published a model which suggested that inhibition of protein synthesis led to the inhibition of DNA synthesis by the depletion of histone from the cytoplasmic pool. This model was simplier than others in that the resulting stabilization of histone mRNA could be viewed as part of the same process that led to its destabilization when DNA synthesis was inhibited. We have developed methodology which allows one to study the flux of histone through the cytoplasm and alterations in that rate of flux when protein or DNA synthesis has been inhibited. These methods can be applied to cells in different states of growth (Gl, GO, and S). Results currently being prepared for publication show that when protein synthesis is inhibited, the rate of DNA synthesis and level of cytoplasmic histone both fall in a biphasic manner with similar kinetics. These results are completely consistent with our proposed model that DNA synthesis can be physiologically limited directly by availability of histone. When DNA synthesis is inhibited, the histone level in the cytoplasmic pool is elevated, with H4 and H3 being elevated more that H2B and H2A. We have also been able to demonstrate that H2B and H2A from prelabeled nuclei can be found in the cytoplasm while H4 and H3 are not. This result suggests that the mechanism is operating in Gl cells as in S cells with inhibited DNA synthesis. Experiments with G0 cells are in progress.
