The major component of cellular RNA is ribosomal RNA. In eukaryotes this RNA contributes 50% - 80% of the total RNA. Ribosomal RNA (rRNA) is required for synthesis of all proteins and its production is closely coordinated with those events that require enhanced translation. The synthesis of rRNA is catalyzed by the DNA-dependent enzyme, RNA polymerase I. The basic hypothesis to be tested in this project is whether production of active RNA polymerase I is required for the mitogen-induced transcription of ribosomal genes (rDNA) in B lymphocytes. This hypothesis will be tested by introduction of anti-RNA polymerase I antibodies into B cells in culture. The ability of these cells to respond to mitogen will then be examined. If the cells containing anti-polymerase antibodies do not respond to mitogen, an important role for active polymerase I in the induction process will be implicated. If so, the mechanism of mitogen-induced production of active polymerase I will be investigated in detail. The role of nonpolymerase factors in mitogen-stimulated rRNA synthesis will also be evaluated. In particular, the presence of rDNA transcription initiation factor(s) will be examined by in vitro transcription assays using truncated genes. In aggregate, these studies will provide fundamental information on the regulation of rRNA synthesis in mammalian cells.