R17 RNA Replicase IV. Rifamycin Sensitive Component of the Replicase *

(1) The growth of the coliphage R17, containing RNA, was examined in E. coli K12, rifamycin-resistant and -sensitive strains. Rifamycin, which is believed to be an antibiotic specific to DNA-dependent RNA polymerase (RNA polymerase) [EC 2.7.7.6], did inhibit the growth of R17 in a sensitive strain but did not interfere with the growth in a rifamycin-resistant strain of E. coli. (2) RNA-dependent RNA polymerase (RNA replicase) was subsequently isolated from E. coli, which had been infected with the R17 phage, and examined for sensitivity to rifamycin in a cell-free system. It was found that the RNA replicase isolated from infected E. coli which was sensitive to rifamycin was also sensitive to rifamycin, but that RNA replicase from resistant E. coli, infected with the wild type R17 phage, showed resistance to rifamycin in a cell-free system. (3) RNA replicase from infected E. coli, either sensitive or resistant to rifamycin, was then dissociated into two components, one coded for by the phage genome (Φ-factor) and the other a host protein(s) containing RNA polymerase (Fraction DR). RNA replicase was then restored by combining the Φ-factor and Fraction DR from different sources and tested for rifamycin sensitivity. It was found that rifamycin had no effect on the Φ-factor, but that Fraction DR contained the site for sensitivity to rifamycin. From these findings, it was confirmed that RNA replicase is composed of protein which is coded for by a phage genome and host protein(s), the latter being the component(s) which is sensitive to rifamycin; most likely this fraction is composed of subunits of RNA polymerase or protein(s) closely associated with it.