Intranuclear Incorporation of Thymic Low Molecular Weight RNA by Murine Bone Marrow Immunoblasts and Inhibition of Plasma Cell Formation by a Derivative of Rifampicin

An in vitro culture system for the proliferation of IgG‐forming plasma cells from mouse bone marrow cultures has previously been described. The present study attempts to elucidate the mode of action of thymic RNA in these cultures. Autoradiography after using radiolabeled thymic RNA showed that radioactive material was mainly incorporated into the nuclei of IgG‐forming plasma cells. No radiolabeled thymic RNA was incorporated into the cells except immunoblasts. The incorporated thymic RNA was acid insoluble and digested by RNase, but resistant to DNase and pronase. Radioactivity in the nucleotide pool after the cells were cultured with radiolabeled thymic RNA was negligible, indicating that reutilization of degraded RNA did not occur in the nuclei of the plasma cells. Moreover, the incorporation of radiolabeled thymic RNA by the cells was not prevented by excess unlabeled nucleosides. Escherichia coli transfer RNA, L‐cell RNA and synthetic polynucleotide poly(A U) were incorporated but were distributed in a different manner in the cells. A derivative of rifampicin, 2′,5′‐dimethyl N(4′)benzyl‐N(4′)[desmethyl]rifampicin (AF/ABDMP), a possible inhibitor of RNA‐dependent DNA polymerases, suppressed both the incorporation of thymic RNA and the differentiation of immunoblasts. AF/ABDMP suppressed DNA synthesis by bone marrow cultures to the same level as those pretreated with anti‐mouse B‐cell antibodies and complement. DNA dependent RNA polymerase activity was observed in the supernatant of bone marrow cultures stimulated by normal syngeneic thymic RNA and human gammaglobulin as antigen. These results imply a possible relationship between B‐cell differentiation and RNA‐dependent DNA polymerases.