Cloning and promoter identification of the iron-regulated cir gene of Escherichia coli.

The cir gene, which encodes the colicin I receptor protein and is regulated by both cellular iron content and growth temperature, was cloned into a multicopy-number plasmid. Physical mapping and complementation analysis established the position of cir between mgl and nfo on the Escherichia coli chromosome. A gene encoding a 32,000-dalton polypeptide was located downstream of and adjacent to cir, but did not appear to be part of the same transcriptional unit. A 525-base-pair fragment from the 5' end of the 1.8-kilobase-pair receptor-coding region directed iron-regulated transcription and translation of a hybrid cir-lacZ gene. Two overlapping promoters were identified by determination of the transcriptional start sites and by sequence analysis. A small open reading frame (120 nucleotides) of unknown significance preceded the receptor-coding sequence. Examination of the amino acid sequence of the receptor purified from the outer membrane revealed that the gene product was processed by removal of a signal peptide and that the mature form had an amino acid sequence near its amino terminus which closely resembled that of several other TonB-dependent proteins.