Replacement of maternal 40S rRNA precursor by newly synthesized precursor in early embryos of Xenopus laevis.

The amount of intact 40S rRNA precursor was followed by Northern hybridization in the course of the early embryogenesis of wild-type Xenopus laevis and its anucleolate mutant. The total amount of 40S rRNA precursor did not alter appreciably until the midblastula stage, decreased at the late blastula stage, and then increased. In the anucleolate mutant, in which no rRNA synthesis occurs, the 40S rRNA precursor decreased at the late blastula stage and disappeared after the gastrula stage. In the nuclear fraction of the wild type, the 40S rRNA precursor was detectable after the midblastula stage. Therefore, the 40S rRNA precursor in the pre-blastula embryos is maternal and decreases at the late blastula stage. New synthesis of 40S rRNA precursor apparently occurs after the midblastula stage.