Uridine and guanosine incorporation by the mouse one-cell embryo

Abstract

ABSTRACT The activity of the embryonic genome prior to the first cleavage has been assessed by studying the uptake of [3H]uridine, its phosphorylation and incorporation into RNA by mouse one-cell embryos. One-cell embryos incorporated [3H]uridine linearly into cold trichloracetic acid (TCA) insoluble material at a low level 1-9 h post fertilization. The incorporation of [3H]guanosine was also low but followed a biphasic curve which had a steeper slope at 1-3 h than during the period 4-9 h post fertilization. Unfertilized mouse ova incorporated very little [3H]uridine or [3H]guanosine into TCA insoluble material, and much of this was RNase insensitive. Dimethyl sulfoxide (DMSO) enhanced the uptake of [3H]thymidine and its incorporation into pronuclear DNA by one-cell embryos, but had no effect on the incorporation of [3H]uridine by them, or of [3H]uridine and [3H]guanosine by unfertilized ova. The uptake and incorporation of [3H]guanosine by one-cell embryos were enhanced by DSMO, but only during the period 1-3 h post fertilization. Sugar derivatives of UDP, and UMP, UDP, UTP, CMP, CDP and CTP have been identified in the soluble fraction obtained from mouse one-cell embryos incubated with [3H]uridine 1-3 h post fertilization. Very little of the [3H]uridine taken up by the embryos is present as [3H]UTP, or [3H]CTP; most is found as [3H]UMP or [3H]UDP or as the sugar derivatives. Alkaline or ribonuclease (A, T1 and T2)hydrolysis of the 3H-labeled ethanol insoluble material precipitated from the lysate of one-cell embryos incubated with [3H]uridine 1-3 h post fertilization liberated radioactive cytidine and uridine-3’-phosphates. This demon strates that [3H]uridine is incorporated into an internal position in RNA and suggests that RNA synthesis does occur in the one-cell embryo 1-3 h post fertilization. Since pronuclei of one-cell embryos incubated with [3H]uridine were not labeled it appears, however, that the RNA synthesized at the one-cell stage is not a product of the embryonic genome.