Cellular regulation of ribosomal DNA transcription: both rat and Xenopus UBF1 stimulate rDNA transcription in 3T3 fibroblasts

A novel RNA polymerase I (RPI) driven reporter gene has been used to invest igate the in vivo role of the architectural ribosomal transcription factor UBF in gene activation and species specificity, It is shown that the level of UBF overexpression in NIH3T3 cells leads to a proportionate increase in the activities of both reporter and endogenous ribosomal genes, Further, co -expression of UBF antisense RNA suppresses reporter gene expression. Thus, UBF is limiting for ribosomal transcription in vivo and represents a poten tial endogenous ribosomal gene regulator In contrast to some in vitro studi es, in vivo, the mammalian and Xenopus forms of UBF1 show an equal ability to activate a mouse RPI promoter. This activity is severely impaired in mut ants compromised for either dimerization or DNA binding. Similarly, the nat ural UBF2 splice variant shows a severely impaired capacity to activate RPI transcription. The data strongly suggest that UBF predominantly regulates ribosomal transcription by binding to and activating the ribosomal genes, b ut does not eliminate a possible secondary role in titrating ribosomal gene repressors such as Rb, Consistent with the DNA folding ability and cellula r abundance of the UBF, we suggest that the protein may regulate a structur al transition between the potentially active and active chromatin states.