Operator-promoter functions in the threonine operon of Escherichia coli.

The prophage curing properties of secondary-site lysogens of coliphage lambda have been studied. The site of integration in the original lysogen (L79) is within the ooerator-promoter region of the thr operon. As a result, expression of the thr enzymes is reduced, and the strain is a leaky threonine auxotroph. Heat pulse curing of strain L79 and a thr+ lysogenic revertant (L79-20) showed that heat pulse curing of both lysogens was int and xis dependent and occurred by correct excisions of the prophage. The heat pulse curing restored strain L79 to prototrophy whereas strain L79-20 synthesized the thr enzymes constitutively and at high levels. This indicates that the reversion mutation in strain L79-20 occurred outside of the prophage and within the operator-promoter region of the thr operon. In contrast, spontaneous curing of both lysogens occurred by both correct and incorrect excisions. Spontaneously cured derivatives of strain L79-20 gave rise to three classes of regulatory mutants affecting operator and promoter functions to the thr operon.