A bacterial RNA polymerase mutant that renders λ growth independent of the N and cro functions at 42°C

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Abstract
We describe a bacterial RNA polymerase mutation, rif 501, which confers rifampicin resistance and thermosensitivity to E. coli K12. The purified RNA polymerase enzyme from rif 501 bacteria shows increased heatsensitivity in vitro at 51°C. However, in vivo, at 42°C, the non-permissive temperature, mutant bacteria continue to grow and to synthesize RNA for 90 min. On a lawn of the mutant bacteria, at 40–41°C, phage λ forms clear plaques (LycA phenotype); this is probably due to an enhancement of cro function; we surmise that at 42°C the transcription originating from the p R (but not from the p L) promoter on the λ genome becomes N-independent and less sensitive to the absence of the cro product. We discuss the possibility that both the N and cro proteins of phage λ interact directly with the bacterial RNA polymerase. These observations indicate that the loss of viability of the rif 501 mutant at the restrictive temperature is not a consequence of an immediate inactivation of RNA polymerase; rather we feel it is due to a modification of the activity of RNA polymerase, leading to a disruption of the cellular regulation.