The incorporation of carbon dioxide into the major classes of RNA during culture of the preimplantation mouse embryo.

The fixation of CO2 into major classes of RNA in the mouse embryo was studied in culture. Total fixation of CO2 was low at the two-cell stage and no label was found in RNA. Between the eight-cell and morula/early blastocyst stages of development, total fixation increased markedly but decreased again at the late blastocyst stage. On a per cell basis, the level of incorporation of CO2 decreased steadily throughout the preimplantation period. A significant acceleration in the accumulation of 14CO2 into all classes of RNA occurred between eight-celled embryos and morulae/early blastocysts, and this effect was more evident when results were calculated in relation to cell number. At the late blastocyst stage, incorporation of label into RNA decreased on a per embryo and a per cell basis. Most of the label from CO2 was incorporated into the r-RNA fraction at all stages of development and incorporation into s-RNA was always less. The pattern of labelling of RNA with 14CO2 was similar to that previously obtained for the incorporation of [3H]uridine into embryonic RNA, suggesting that most of the CO2 entering the RNA pool may be incorporated into nucleotide bases. The s-RNA and r-RNA fractions were susceptible to digestion with both pancreatic ribonuclease and 0-3 M alkali. Approximately 31% of the label in the TD-RNA fraction remained after hydrolysis with ribonuclease and a similar proportion of the TD-RNA was resistant to alkali treatment. Incorporation of CO2 by morulae/early blastocysts was substantial during culture in substrate-free medium but was increased significantly in medium containing lactate plus pyruvate. Carbon dioxide fixation into RNA was decreased by preculture for 48 hr before incubation in radioactive medium. When compared with freshly collected morulae/early blastocysts, the proportion of the total label in the s-RNA fraction of precultured embryos was low, and a correspondingly greater proportion of the total label was found in the TD-RNA fraction.