A temperature-sensitive mutant of E. coli is described. At the nonpermissive temperature, the capacity for RNA and protein synthesis decreases logarithmically in the mutant. The mutant is unable to support the growth of f2 or T7 virus, even at the permissive temperature. The temperature-sensitive mutation maps approximately 1′ away from rifr in E. coli and therefore affects a gene previously undescribed. The temperature sensitivity is suppressed by sublethal concentrations of rifampicin. Moreover, in rifr Ts double mutants, the Ts mutation suppresses rifr and vice versa. The partially purified RNA polymerases from mutant and wild-type cells have different temperature and salt optima.