ThehtrMgene, whose product is essential forEscherichia coliviability only at elevated temperatures, is identical to therfaDgene

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Abstract
We have identified a new E. coll gene, htrM. The htrM gene was identified because its insertional inactivation by the Tn5 transposon results in E. coli's inability to form colonies at temperatures above 43°C. The corresponding htrM+ gene was cloned on the basis of its ability to correct the temperature-sensitive phenotype of the htrM: Tn5 insertion mutations. The htrM gene has been mapped to 81.2 min on the conventional E. coli genetic map. It was sequenced and shown to code for an acidic, 34,893-Da polypeptide. Three transcriptional starts were located 48, 90 and 123 nucleotides upstream of the ATG, initiation codon referred to as the P1, P2 and P3(hs) promoters, respectively. The −10 and −35 regions of the P1 promoter bear a close similarity to the Eα70-recognized consensus sequences, while the −12 region of the P2 promoter resembles the consensus promoter sequence transcribed by the rpoN gene product. Transcripts of the htrM gene accumulate with increasing temperature. The −10 and −35 regions of the P3(hs) promoter, represented by nucleotides 160 to 130 upstream of the ATG initation codon, are similar to the Eα32-recognized consensus sequences. The α32 transcription factor is essential for maximal htrM gene transcription, since htrM RNA transcripts are made at reduced rates in a rpoH null mutant background. Surprisingly, the htrM gene turns out to be identical to rfaD, whose product is required for the biosynthesis of the ADP-L-glycero-D manoheptose lipopolyaccharide precursor [Pegues et al. (1990) J.Bacteriol. 172, 4652-4660].