The Xer activation factor of TLCΦ expands the possibilities for Xer recombination

Many mobile elements take advantage of the highly-conserved chromosome dimer resolution system of bacteria, Xer. They participate in the transmission of antibiotic resistance and pathogenicity determinants. In particular, the toxin-linked cryptic satellite phage (TLCΦ) plays an essential role in the continuous emergence of new toxigenic clones of the Vibrio cholerae strain at the origin of the ongoing 7th cholera pandemic. The Xer machinery is composed of two chromosomally-encoded tyrosine recombinases, XerC and XerD. They resolve chromosome dimers by adding a crossover between sister copies of a specific 28 base pair site of bacterial chromosomes, dif . The activity of XerD depends on a direct contact with a cell division protein, FtsK, which spatially and temporally constrains the process. TLCΦ encodes for a XerD-activation factor (XafT), which drives the integration of the phage into the dif site of the primary chromosome of V. cholerae independently of FtsK. However, XerD does not bind to the attachment site ( attP ) of TLCΦ, which raised questions on the integration process. Here, we compared the integration efficiency of thousands of synthetic mini-TLCΦ plasmids harbouring different attP sites and assessed their stability in vivo . In addition, we compared the efficiency with which XafT and the XerD activation domain of FtsK drive recombination reactions in vitro . Taken together, our results suggest that XafT promotes the formation of synaptic complexes between canonical Xer recombination sites and imperfect sites. ### Competing Interest Statement The authors have declared no competing interest.