Distribution of ribosomal cistrons in the nuclei of Ehrlich ascites tumor and hepatic cells from mice.

Previously, we found that gene expressions for ribosomal RNA differ in tumorous and normal cells, so we compared the distributions of ribosomal cistrons in subfractions of nuclei from Ehrlich ascites tumor and mouse liver cells. The mean number of nucleoli per nucleus is mouse liver (diploid cells) is 3.7 while, in hyperdiploid and hypotetraploid strains of Ehrlich ascites tumors, the means are 1.8 and 3.6, respectively. However, in spite of the relatively few nucleoli per nucleus in tumor cells, the amount of DNA in the nucleoli is increased significantly (threeto fourfold). The ratio of annealing of radioactive ribosomal RNA with nucleolar DNA was similar in tumor cells and mouse liver cells, so the total amount of ribosomal genes in individual nucleoli of tumor cells must be greater than in mouse liver cells, and this increase may be enough to compensate for the reduction of nucleolar number in the nuclei of tumor cells. It is probable that fusion of nucleolar organizers (rather than amplification of ribosomal genes) occurs in the nucleoli of tumor cells, because no significant difference was found in the numbers of ribosomal cistrons in whole nuclear DNA of tumor and normal liver cells. Several ribosomal genes in extranucleolar regions are not expressed in vivo but were detected by hybridization in vitro.