Regulation of ribosomal RNA and proteins in mouse-hamster hybrid cells.

We examined the synthesis and accumulation of ribosomal RNA and protein in hybrid cells formed by fusing mouse 3T3 cells with Chinese hamster ovary cells resistant to the antibiotic emetine. These hybrid lines retained chromosomes of both parents and accumulated mouse and hamster ribosomal components in hybrid ribosomes. “he synthesis of precursor ribosomal RNA was found to vary from half-hamster in one line to virtually all mouse in another, although the ribosomal DNA of both mouse and hamster were present in all hybrids examined. In each line the mouse: hamster ratio of precursor RNA was identical to that found in stable ribosomes, demonstrating that the processing of precursor RNA was independent of its species. In each hybrid examined, the ribosomal proteins of both species were synthesized in equal amounts and were utilized with equal efficiency in the assembly of ribosomes. The synthesis of ribosomal proteins relative to other cellular proteins was found to be identical to the hybrid and parental cells. Ribosomal proteins accumulated in the hybrids to the same level as in parental cells. The synthesis and accumulation of the individual ribosomal protein responsible for emetine resistance was examined in a hybrid expressing both mouse and hamster varieties of this protein, and in an emetineresistant subclone expressing only the hamster variety. In both lines the synthesis and accumulation of this protein was identical, suggesting that the synthesis of ribosomal proteins was independent of the difference in gene dosage. The quantitative synthesis of ribosomes was found to be similar in hybrid and parental lines despite differences in chromosome content.