A MUTATION IN THE RPOA GENE ENCODING THE ALPHA-SUBUNIT OF RNA-POLYMERASE THAT AFFECTS METE-METR TRANSCRIPTION IN ESCHERICHIA-COLI

The DNA-binding protein MetR belongs to the LysR family of transcripti onal activators and is required for expression of the metE and metH pr omoters in Escherichia coli. However, it is not known if this activati on is mediated by a direct interaction of MetR with RNA polymerase. In a search for RNA polymerase mutants defective in MetR-mediated activa tion of the metE gene, we isolated a mutation in the a subunit of RNA polymerase that decreases metE expression independently of the MetR pr otein. The mutation does not affect expression from the metH promoter, suggesting that the alpha subunit of RNA polymerase interacts differe ntly at these two promoters. The mutation was mapped to codon 261 of t he rpoA gene, resulting in a change from a glutamic acid residue to a lysine residue. Growth of the mutant is severely impaired in minimal m edium even when supplemented with methionine and related amino acids, indicating a pleiotropic effect on gene expression. This rpoA mutation may identify either a site of contact with an as yet unidentified act ivator protein for metE expression or a site of involvement by the alp ha subunit in sequence-specific recognition of the metE promoter.