Transcription of mouse rDNA is regulated by an activated subform of RNA polymerase I

We have identified the species-nonspecific factor required for mouse rDNA transcription, factor C, as an activated subform of RNA polymerase I. C is an RNA polymerase I since it copurifies with bulk polymerase I activity on the three chromatographic columns used to achieve a virtually homogenous preparation of polymerase I, as well as on four additional matrices; it is quantitatively neutralized as well as immunoprecipitated by two different types of anti-polymerase I antibodies; and it has thermal lability identical to that of bulk polymerase I. However, C is clearly distinct from bulk polymerase I in its ability to participate in the stable rDNA transcription complex and to catalyze accurate initiation of rRNA synthesis. It also has a greater sedimentation coefficient than bulk polymerase I. Furthermore, this activated polymerase subform is specifically lacking in extracts of cells in which rDNA transcription was down-regulated because of cycloheximide treatment or attainment of stationary phase. These data suggest that regulation of rDNA transcription in vivo may involve modulation in availability of the activated polymerase I subform.