rDNA transcription and pre-rRNA processing during the differentiation of a mouse myoblast cell line

The rates of formation and processing of rRNA and transcription of rDNA were examined during the differentiation of MM14DZ mouse myoblasts into muscle fibers. Analyses of the incorporation of [3H]uridine into 18 and 28 S rRNAs and the UTP precursor pool indicate that the rate of rRNA formation is 2.3-fold higher in myoblasts as compared to muscle fibers. To determine if the rate of rRNA formation is regulated at the level of pre-rRNA processing, the labeling kinetics and steady state levels of pre-rRNAs were measured in myoblasts and fibers. These experiments suggest that the time required for and the efficiency of pre-rRNA processing are the same in myoblasts and fibers, but that the steady state levels of all pre-rRNAs detected (32, 34, 37, 41, and three 45 S subspecies) are reduced 2.5- to 3.0-fold in fibers. This suggests that the reduced rate of rRNA formation in fibers is not regulated at the level of pre-rRNA processing but is due to a decrease in the rate of rDNA transcription. The transcriptional rates for rRNA in myoblasts and fibers were calculated from measurements of the total rate of transcription and from hybridization analyses of radioactive rRNA formed in isolated nuclei or in short pulses of cultured cells. These experiments indicate that the rates of 18 and 28 S rDNA and 5 S rDNA transcription are reduced 2.5 to 3.0-fold in fibers. Thus, a coordinate change in the rate of rDNA transcription is the primary mechanism regulating the formation of rRNA during the differentiation of this myoblast line. This decreased transcription in fibers appears to be due to modulation of the activity of the known mouse rDNA promoter, as S1 nuclease mapping experiments indicate that rDNA transcription initiates at the same location in both myoblasts and fibers.