The right end of MudI(Ap,lac).

Stable derivatives of the bacteriophage MudI-(Ap,lac) were used to generate operon fusions in S. typhimurium which exhibit a sectoring phenotype with respect to lacZ expression. The Lac- to Lac+ conversion was shown to be the result of small deletions involving the right end of the Mud/I element. DNA sequence analysis of several different fusions revealed that this end of MudI(Ap,lac) contains an assymetric inverted repeat of the attR site found in the wild-type Mu phage. A model is presented which explains how such a structure was formed in the construction of MudI(Ap,lac). In addition, this model explains the observed deletion formation and the Lac- to Lac+ conversion in the sectoring fusions.