A rifampin-resistant ribonucleic acid (RNA) polymerase mutant, rift5l, derived from a presumptive RNA synthesis mutant of Escherichia coli K-12, complements rifr RNA polymerase mutants isolated from other strains of E. coli K-12.A temperature-sensitive (TS) mutant of Escherichia coli K-12, T8-290, is probably defective for ribonucleic acid (RNA) synthesis (8). It has no known mutations other than the one leading to temperature sensitivity. A relation between the 1 mutation and rifampin resistance (rifr) genes of E. coli has established; namely, when a rifr mutation isolated from a temperature-independent (Ti) strain is transferred to 78-290, each mutation suppresses the other (8). This suppression is observed with both the classical RNA polymerase (9) and the so-called "permeability" ritr muta tions (8), although the phenotypes are different in the two cases. T7rifr (polymerase) double mutants are as weakly ritr and almost as TS as T'-290, whereas Trifr ("permeability") double mutants are more rifr and are T1 (S. Marshall, unpublished data).Since the usual ritr mutations appear not to be fully expressed in 7'-290, the possibility arose that fully rift strains, isolated by mutation from P-290, might differ from those isolated from other K-12 strains. In this paper rift mutants that are not permeability mutants, as defined by loss of resistance of ethylenediaminetetraacetic acid-treated cells (8), are assumed to be mutants with altered RNA polymerase. Mutations in new cistrons can be detected by genetic complementation tests; partial diploids carrying ritr mutations in different cistrons should exhibit a rifampin-sensitive (rif) phenotype while those carrying rift mutations in the same gene show a ritr pheno-'Present addreas: Gerontology Research Center, Baltimore City Hospitals, Baltimore, Md. 21224. type. Indeed, rift ("permeability")/rift (polymerase) heterogenotes are phenotypically rifp