On the Mu Repressor and Early DNA Intermediates of Transposition

unique in that they replicate DNA by transposition (for review, see Toussaint and Resibois 1983). During lytic growth, about 100 new viral copies are placed ran-domly in the host chromosome. These viruses also in-tegrate linearly into the chromosome and establish a lysogenic state by producing a repressor. Their ability to change efficiently from the integrated stable state of the lysogen to the rapid replicative cycle makes them powerful systems for biochemical analysis of transposition. The transition from a lysogenic state to the lytic cycle requires activation of a Mu promoter and synthesis of Mu-encoded proteins. To understand the early events in this transition, we have studied the Mu repressor and the early transposition reaction products. Purified