The L-arabinose operon in Escherichia coli B-r: a genetic demonstration of two functional states of the product of a regulator gene.

The product of the regulator gene araC in the L-arabinose gene complex exists in two functional states: P1, the repressor, and P2, the activator, presumably in equilibrium with each other, and with P1 and P2 attached to their respective controlling sites, araO, the operator, and araI, the initiator. The controlling sites are linked in the following order with respect to genes araB and araC: BIOC. Two C gene deletions (Delta719 and Delta766) serve to define the newly described araO site, and to place it adjacent to the left end of the C gene. We have suggested that deletion 719 deletes, in addition to the C gene, part or all of the araO site. Deletion 766 leaves the araO site intact. Complementation analysis employing stable merodiploids indicates that the repressor-operator site function is epistatic over the activator-initiator site function. It is necessary both for activator (P2) to be present and for repressor (P1) to be absent at their respective controlling sites (araI and araO) for full expression of the L-arabinose operon.