Mu Transposition in the Absence of the Target-capture Protein MuB Reveals New Roles of MuB in Target Immunity and Target Selection, and Redraws the Boundaries of the Insular Ter Region of E. coli

The target capture protein MuB is responsible for the high efficiency of phage Mu transposition within the E. coli genome. However, some targets are off-limits, such as regions immediately outside the Mu ends (cis-immunity) as well as the entire ~37 kb genome of Mu (Mu genome immunity). Paradoxically, MuB is responsible for cis-immunity and is also implicated in Mu genome immunity, but via different mechanisms. In this study, we tracked Mu transposition from six different starting locations on the E. coli genome, in the presence and absence of MuB. The data reveal that the ability of Mu to sample the entire genome during a single hop in a clonal population is independent of MuB, and that MuB is responsible for cis-immunity, plays a lesser role in Mu genome immunity, and facilitates insertions into transcriptionally active regions. Unexpectedly, transposition patterns in the absence of MuB have helped extend the boundaries of the insular Ter segment of the E. coli genome.