RoleofDNA-Dependent RNA Polymerase IIIintheTranscription of thetRNAand5SRNA Genes (mousemyelomacells/isolated nuclei/a-amanitin)

ABSTRACT Mousemyelomacellshavepreviouslybeenshown (L. B. Schwartz,V. E. F. Sklar, J. A. Jaehning, R.Weinmann &R. G. Roeder,submittedforpublication)tocontain two chromatographically distinct forms of RNApolymeraseIII (designatedIIIAand IIIB). Theenzymesareunaffected bylowa-amanitin concentrations whichcom-pletely inhibit RNA polymerase II, but they exhibitcharacteristic inhibition curves (identical for IIIA and IIIB) at higher toxin concentrations. RNApolymerase Iwas unaffected at all a-amanitin concentrations tested.MyelomaRNApolymerases II, ITI, and IlB appeartobeinhibitedbythesamemechanism,sincethetoxin rapidlyblocks chain elongation by each enzyme. The charac-teristic a-amanitin sensitivity of RNA polymerase IIIhas been employed in studies of the function(s) of theclass III RNApolymerases.Isolated myeloma nuclei and nucleoli continue tosynthesize RNA via the endogenous RNA polymeraseswhenincubated in vitro. With nuclei, newly synthesized4Sprecursor (pre-4S) and 5S RNAspeciesweredetectedby