Does the rate of ribosomal RNA synthesis vary depending on the number of nucleoli in a nucleus?

Cultured kidney cells of Xenopus laevis were pulse-labeled with [ 3 H]uridine for 10, 20 and 30 min during their logarithmic growth phase and then processed for autoradiography. The labeled cells were assigned into two categories, one- and two-nucleolated cells, and the rate of ribosomal RNA (rRNA) synthesis was measured by counting the number of grains in nucleoli. The results obtained revealed that a two-nucleolated cell incorporated significantly much more radioactivity into its nucleoli than did a one-nucleolated partner for all the periods examined. Cells of these different nucleolar types, however, contained essentially the same amount of rDNA (DNA complementary to rRNA) as estimated by in situ hybridization with [ 125 I]rRNA. Although it remains to be proved that the observed increase in incorporation represents the increased rate of rRNA synthesis in two-nucleolated cells, the present findings seem to be very interesting, since they might indicate that the activity of rRNA genes is in some way regulated or affected by their spatial relationship in a cell nucleus.