Synthesis of DNA, rRNA, and protein by Rickettsia prowazekii growing in untreated or gamma interferon-treated mouse L929 cells

The syntheses of DNA, rRNA, and protein by Rickettsia prowazekii growing in mouse fibroblastic L929 cells were measured at various times after the addition of gamma interferon (IFN-gamma) to correlate the inhibition of a site of macromolecular synthesis with the established IFN-gamma-induced inhibition of rickettsial growth. A method was developed to measure the syntheses of DNA, rRNA, and protein by R. prowazekii during a 2-h pulse-labeling period while the rickettsiae were growing within cultured host cells that had intact macromolecular synthesis. This method involved incubation of the rickettsia-infected cells with a radioactive precursor (H3 32PO4 or Tran35S-label), purification of the rickettsiae, purification of rickettsial nucleic acids, and analysis of rickettsial nucleic acids and proteins by electrophoresis and autoradiography. A key feature of the method involved the use of calculated specific activities from a densitometric analysis of gels and autoradiograms, a procedure that made the data independent of rickettsial recovery. Rickettsial DNA and rRNA syntheses were both inhibited 12 h after the addition of IFN-gamma to infected cultures, whereas the synthesis of rickettsial proteins was not inhibited at this time. In contrast, at 20 h after the addition of IFN-gamma, rickettsial DNA, rRNA, and protein syntheses were all inhibited.