Transcriptional regulation of mammalian ribosomal RNA genes

Structure of the core promoter of human and mouse ribosomal RNA gene. Asymmetry of species-specific transcription

In vitro transcription of the ribosomal RNA gene (rDNA) shows a remarkable species specificity such that human and mouse rDNA cannot use heterologous extracts of each other. The region that is responsible for this specificity has been studied using human-mouse chimeric genes and characteristic structures of both core promoters are presented. When the mouse sequence is substituted by the corresponding human sequence from upstream, the promoter activity in the mouse extract begins to decline at nucleotide -32 or -30, decreasing gradually and is lost completely at -19. A similar gradual decrease was noted for the 3' side substitution, which started at nucleotide -14 and was completed when up to the nucleotide -22 mouse position was replaced by the corresponding sequence from human. Thus, in the mouse rDNA core promoter, the sequence that is involved in species specificity resides only in a stretch encompassing the non-conserved region between the distal conserved sequence (DCS) and the proximal conserved sequence (PCS), plus two altered nucleotides in the PCS. When human rDNA is transcribed with human cell extract, the mouse sequence cannot substitute for the human sequence within the region from nucleotide -43 to +17 without affecting promoter activity significantly. This asymmetry of species specificity is due to the presence of nucleotides -43, +1 and +17, which are sensitive to change in only the human core promoter. The difference in the 5' border is ascribed to the species specificity of a transcription factor TFID, which recognizes this region. But the large difference of the 3' border is apparently due to another factor, possibly RNA polymerase I itself, because this region is not recognized by TFID in either human or mouse. Mammalian rDNA core promoter appears to consist of a tandem mosaic in which three evolutionarily conserved sequences alternate with non-conserved sequences having certain functionally important nucleotides. Not only non-conserved sequences and non-conserved nucleotides in conserved sequences, but also the spacings between the three conserved regions, play a crucial role in species specificity.

Transcription factors mediate rRNA synthesis during myogenesis

This study was designed to identify factors which control rRNA transcription during terminal differentiation of rat L6 myoblasts. A cell-free system using 100,000 x g supernatants (S100 extracts) from rat L6 myoblasts and a genomic rRNA clone was established which accurately transcribes the rat rRNA gene. The myoblast S100 extract produced a high level of transcription with accurate initiation at the rRNA promoter. Myotube S100 extracts, on the other hand, displayed a reduced capacity to transcribe the rRNA gene. Thus constituents of the cell extract, rather than rearrangements of the DNA sequences, are implicated in regulating rRNA gene expression. The RNA polymerase content of the S100 extracts was analyzed using alpha-amanitin to determine the activity of each polymerase species and found to be comparable in both myoblast and myotube S100 extracts. This observation suggests that factors other than RNA polymerase I are involved in regulating rRNA transcription. Two of the factors associated with accurate rRNA gene expression in vitro, namely transcription factors B and D, occurred at comparable levels in both myoblast and myotube extracts suggesting that other components of the transcription complex are responsible for limiting rRNA transcription in myotubes. The cell-free system should facilitate identification and characterization of these factors which regulate expression of the rRNA gene.