Xer1-Mediated Site-Specific DNA Inversions and Excisions in Mycoplasma agalactiae

Surface antigen variation inMycoplasma agalactiae , the etiologic agent of contagious agalactia in sheep and goats, is governed by site-specific recombination within thevpma multigene locus encoding the Vpma family of variable surface lipoproteins. This high-frequency Vpma phase switching was previously shown to be mediated by a Xer1 recombinase encoded adjacent to thevpma locus. In this study, it was demonstrated inEscherichia coli that the Xer1 recombinase is responsible for catalyzingvpma gene inversions between recombination sites (RS) located in the 5′-untranslated region (UTR) in all sixvpma genes, causing cleavage and strand exchange within a 21-bp conserved region that serves as a recognition sequence. It was further shown that the outcome of the site-specific recombination event depends on the orientation of the twovpma RS, as direct or inverted repeats. While recombination between invertedvpma RS led to inversions, recombination between direct repeatvpma RS led to excisions. Using a newly developed excision assay based on thelacZ reporter system, we were able to successfully demonstrate under native conditions that such Xer1-mediated excisions can indeed also occur in theM. agalactiae type strain PG2, whereas they were not observed in the controlxer1 -disrupted VpmaY phase-locked mutant (PLMY), which lacks Xer1 recombinase. Unless there are specific regulatory mechanisms preventing such excisions, this might be the cost that the pathogen has to render at the population level for maintaining this high-frequency phase variation machinery.