An Escherichia coli mutant with a temperature-sensitive function affecting bacteriophage Qbeta RNA replication.

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Abstract
An E. coli mutant capable of supporting replication of bacteriophage Q.beta. at 33.degree. C, but not at 40.degree. C, was isolated. Coliphages f2, R23, fd and .lambda. formed plaques on mutant cells at both temperatures. Temperature-shift experiments showed that bacteriophage Q.beta. replication was blocked in the mutant within the first 20-30 min of infection. The defect did not prevent translation of the Q.beta. polymerase gene or assembly of catalytically active Q.beta. replicase molecules. Mutant cells infected at 40.degree. C hyperinduced replicase active in vivo and in vitro. Zone sedimentation of the in vivo RNA product showed it to consist of partially double-stranded material sedimenting at 9 S, with little or no viral 32S RNA. The 9S RNA was also found, along with a predominant peak of 32S RNA, in parental cells infected at 40.degree. C, but not in cells infected at 33.degree. C. The temperature-sensitive component is thus required for viral RNA replication, but not for other RNA synthesis catalyzed by the replicase. Uninfected mutant cells grew normally at 40.degree. C in nutrient broth, but not in glucose- or glycerol-minimal media. Revertants selected for their ability to grow in minimal medium at 40.degree. C also supported bacteriophage Q.beta. replication at 40.degree. C.