A Mutation in therpoAGene Encoding the aSubunit of RNA Polymerase That AffectsmetE-metR Transcription inEscherichia coli

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The DNA-binding protein MetR belongs to the LysR family of transcriptional activators and is required for expression of the metE and metH promoters in Escherichia coli. However, it is not known if this activation is mediated by a direct interaction of MetR with RNA polymerase. In a search for RNA polymerase mutants defective in MetR-mediated activation of the metE gene, we isolated a mutation in the a subunit of RNA polymerase that decreasesmetEexpression independently of the MetR protein. The mutation does not affect expression from themetHpromoter, suggesting that the asubunit of RNA polymerase interacts differently at these two promoters. The mutation was mapped to codon 261 of therpoAgene, resulting in a change from a glutamic acid residue to a lysine residue. Growth of the mutant is severely impaired in minimal medium even when supplemented with methionine and related amino acids, indicating a pleiotropic effect on gene expression. ThisrpoAmutation may identify either a site of contact with an as yet unidentified activator protein for metE expression or a site of involvement by the a subunit in sequence-specific recognition of the metE promoter.

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