ONSET OF RNA-SYNTHESIS AND POLY(A) CONTENT OF EARLY RABBIT EMBRYOS - COMPARISON WITH SHEEP

RNA synthesis in 2-32 cell embryos, as assessed by alpha-amanitin-sens itive H-3-uridine incorporation, was first detectable in 4-cell stage rabbit and 8-cell stage sheep embryos. In the rabbit, uridine incorpor ation was detectable at the 2-cell stage but was unaltered by alpha-am anitin, indicating synthesis of non-polymerase II-dependent RNA specie s. Initiation of mRNA synthesis as determined by in situ hybridization with H-3-poly (U) probe was first detectable in late 2-cell stage rab bit and 4-cell stage sheep embryos. In the rabbit embryos, nuclear lab elling increased from the late 2-cell stage to the 16-cell stage, foll owing a pattern similar to that of 3H-uridine incorporation. In contra st, the intensity of cytoplasmic labelling decreased from the 1- to th e 8-cell stage and then increased up to the 32-cell stage. In sheep em bryos, nuclear labelling by the poly (U) probe increased from the 4- t o the 16-cell stage. It is concluded that initiation of transcription of the embryonic genome (mRNA) can be detected via the current methods used at the 4-cell stage in the rabbit and the 8-cell stage in the sh eep.