Determination of the rate of rRNA synthesis in Xenopus laevis triploid embryos produced by low-temperature treatment.

Triploid embryos of Xenopus laevis were obtained by cold-temperature shocking of the fertilized eggs, and the rate of the ribosomal RNA (rRNA) synthesis was determined for comparison with that in diploid embryos. For this purpose, both triploid and diploid embryos were dissociated into cells at the neurula stage, and then labeled with (3H)uridine for varying lengths of time. The rate of rRNA synthesis, as estimated after determination of (3H)UTP specific radioactivity and the total label incorporation into the purified rRNA, was about 0.1 pg/cell/hr for both diploid and triploid embryo cells. Nuclei of triploid embryo cells contained three nucleoli of apparently similar sizes--an indication of the functioning of all the three rRNA gene clusters to a more or less similar extent. Also, rates of synthesis of 4S RNA and 5S RNA were determined: Both rates did not change appreciably between triploid and diploid embryo cells. Based on these results, it appears that transcription of these redundant genes occurs at a constant rate on a per cell basis irrespective of the presence of 1.5 times as many genes as the control.