| Abstract: | The effect of lipopolysaccharide on RNA polymerase I activity in primary cultures of murine B lymphocytes has been examined. In cells treated with mitogen for 48 h, the activity of RNA polymerase I was approximately 15 times greater than in control cells. In situ localization of RNA polymerase I using indirect immunofluorescence indicated that there was at least a 10-fold increase in the amount of this enzyme associated with nucleoli of 48 h mitogen-treated cells relative to control cells. Immunoblotting experiments demonstrated a similar increase in the concentration of the 190-kDa subunit bound to DNA; the concentrations of the other polymerase I-associated polypeptides did not correlate with rRNA synthesis. Assuming 1 mol of the 190-kDa polypeptide/mol of polymerase I, it was estimated that 2,300 and 30,000 molecules of enzyme were associated with rDNA in the unstimulated and stimulated B cell, respectively. Thus, an increased cellular concentration of the 190-kDa subunit of RNA polymerase I and its association with ribosomal DNA may be a crucial step in rRNA synthesis. |
