Significance of constitutive integrase synthesis.

One conceivable function for constitutive integrase formation by lambda prophage is to stabilize the inserted state by catalyzing reinsertion of prophages that are accidentally excised. As this hypothesis implies a dynamic equilibrium betwen inserted and noninserted DNA, the existence of such an equilibrium is explored. By examining the frequency with which prophages appear in an initially unoccupied chromosomal site of a lysogenic bacterium in which the prophage attachment site is duplicated, the off-rate is estimated as less than 10(-2) per generation for wild-type lambda, and less than 4 x 10(-4) for N- mutants of lambda. From the rate of integrase-catalyzed haploidization of certain partial diploid strains, the rate of spontaneous integrase activity is estimated as 3 x 10(-3) per generation. From these values I conclude that constitutive integrase will not appreciably stabilize the inserted state by virtue of its known activity.