Control of protein synthesis during early cleavage of sheep embryos

Sheep embryos, radiolabelled with [35S]methionine, were studied during the first four cell cycles after fertilization to determine the stage at which the regulation of protein synthesis switches from the direction by maternal mRNA to mRNA transcribed by the embryonic genome. Total protein synthesis was consistently high during the first 2 cleavage divisions, dropped by 95% in the 3rd cell cycle, remained low in the 4th and increased again in the 5th cycle. A consistent pattern of proteins was synthesized during the first 3 cell cycles after fertilization followed by major changes in subsequent cycles. The inhibition of transcription by alpha-amanitin, an inhibitor of polymerase II, did not affect cleavage or protein synthesis during the first 3 cell cycles but blocked cleavage and protein synthesis thereafter. The results indicate that the full activation of transcription in sheep embryos occurs in the 4th cell cycle.