Precursor pools and RNA synthesis in preimplantation mouse embryos

Abstract

The increase in incorporation of uridine into RNA between days 2 and 3 of preimplantation mouse embryo development has previously been shown to be accompanied by a parallel increase in the uptake of the precursor by the embryos, and similar results are also obtained with adenosine. However, when guanine or adenine are used as precursors, the changes in uptake and incorporation into RNA are no longer coordinate, demonstrating that the change in incorporation is not merely a reflection of changes in precursor uptake. The relationship of the uptake of adenine and adenosine to the incorporation into RNA, at different concentrations of the substrates in the incubation medium, was investigated in day 3 embryos. The results indicated that the internal precursor pool was approaching saturation at the highest substrate concentrations used and allowed for calculation of a corrected value for precursor incorporation into RNA. Since the measured ATP content of the embryos was considerably greater than the total amount of precursor estimated to be present as ATP in the day 3 embryos, it is concluded that much of the embryonic ATP is not available for RNA synthesis. With guanine as precursor on day 3 and with all precursors on day 2, no evidence was obtained for saturation of the internal precursor pools, thereby precluding calculation of the actual amount of precursor incorporated into RNA. However, taking into account the measured incorporation of precursors into RNA and the nature of the pools which the precursors are entering, it is concluded that RNA synthesis per cell between days 2 and 3 of embryo development either remains constant or, at most, increases by only a modest amount.