Exonuclease activity that degrades histone mRNA is stable when DNA or protein synthesis is inhibited

The induction and repression of histone synthesis during the cell cycle are regulated, in part, by modulating histone mRNA stability. When DNA synthesis stops, histone mRNA seems to be destabilized, perhaps via an autoregulatory circuit triggered by cytoplasmic histones. We have used an in vitro mRNA decay system to determine whether differential histone mRNA turnover is linked to changes in the basal activity of cytoplasmic mRNA-degrading enzymes. The basal level of the polysome-associated exonuclease enzyme or enzymes that degrade histone mRNA was similar in untreated cells and in cells exposed to DNA or protein synthesis inhibitors. Histone mRNA decay was accelerated in reactions supplemented with histones and soluble cytoplasmic factor(s) (S130), but S130s from control and inhibitor-treated cells were indistinguishable in these assays. The data indicate that basal exonuclease activity is stable or constitutive. The putative factor(s) required for autoregulating histone mRNA decay also do not change appreciably when DNA or protein synthesis is inhibited. The implications of these results with regard to the autoregulation of histone mRNA turnover are discussed.