The Escherichia coli L-arabinose operon: binding sites of the regulatory proteins and a mechanism of positive and negative regulation.

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Abstract
The locations of DNA binding by the proteins involved with positive and negative regulation of transcription initiation of the L-arabinose operon in E. coli were determined by the DNase I protection method. Two cyclic AMP receptor protein sites were found, at positions -78 to -107 and -121 to -146, an araC protein-arabinose binding site was found at position -40 to -78 and an araC protein-fucose binding site was found at position -106 to -144. These locations, combined with in vivo data on induction of the 2 divergently oriented arabinose promoters, suggest the following regulatory mechanism: induction of the araBAD operon occurs when cyclic AMP receptor protein, araC protein and RNA polymerase are all present and able to bind to DNA. Negative regulation is accomplished by the repressing form of araC protein binding to a site in the regulatory region such that it simultaneously blocks access of cyclic AMP receptor protein to 2 sites on the DNA, 1 site of which serves each of the 2 promoters. From a single operator site, the negative regulator represses the 2 outwardly oriented ara promoters. This regulatory mechanism explains the known positive and negative regulatory properties of the ara promoters.