By Northern blotting methods we have recently found that the relative amount of histone H4 mRNA per Xenopus embryo is constant during cleavage, doubles during the gastrula stage, then decreases at the neurula stage to the level slightly lower than that of the cleavage‐embryo. At each developmental stage, the level of mRNA will depend on three parameters: amount of maternal mRNA remaining, rate of the synthesis of the mRNA and rate of the decay of the mRNA. In the present experiment, we compared rates of the decay of H4 mRNA in Xenopus embryonic cells at different stages by pulse‐chase experiments with actinomycin D as an inhibitor of transcription. Under the conditions used, the half‐life of H4 mRNA was estimated to be 90, 80, and 100 min, for the late blastula, late gastrula and neurula stages, respectively. The decay of H4 mRNA with a half‐life of 90 min at the late blastula stage predicts exhaustion of most of maternal H4 mRNA by the early gastruia stage. The slightly longer half‐life of H4 mRNA in neurula than in late gastrula cells suggests that H4 mRNA is more stable in neurula cells than in late gastrula cells, and therefore, the large decrease in the level of H4 mRNA observed at the neurula stage does not depend on the increase in the turnover‐rate of H4 mRNA. Probably, neurula cells synthesize less H4 mRNA than late gastrula cells but utilize it for longer time.
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