Comparison of in vivo and in vitro ribosomal RNA synthesis in nucleolar mutants of Xenopus laevis.

Cells of embryos carrying a lethal nucleolar mutation have been maintained in vitro for extended periods of time. Normally these mutants live only 9 to 12 days after fertilization but their cells in culture will survive for more than 3 months. The extent of ribosomal RNA (rRNA) synthesis was determined in primary cultures prepared from normal embryos and nucleolar mutants having different numbers of ribosomal RNA genes. We found that the accumulation of radioactivity into rRNA for normal and mutant embryos was similar in vivo and in vitro. In primary cultures of normal embryos which have two nucleoli per cell and mutant embryos which have only one nucleolus per cell, the incorporation of radioactivity into rRNA was similar even though the normal cells have twice as many rRNA genes. Thus the mechanism which regulates dosage compensation of the rRNA genes operates both in vivo and in vitro.