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Antigenic variation plays a vital role in the pathogenesis of many infectious bacteria and protozoa including  Borrelia burgdorferi , the causative agent of Lyme disease. VlsE, a 35 kDa surface-exposed lipoprotein, undergoes antigenic variation during  B. burgdorferi  infection of mammalian hosts, and is believed to be a critical mechanism by which the spirochetes evade immune clearance. Random, segmental recombination between the expressed  vlsE  gene and adjacent  vls  silent cassettes generates a large number of different VlsE variants within the infected host. Although the occurrence and importance of  vlsE  sequence variation is well established, little is known about the biological mechanism of  vlsE  recombination. To identify factors important in antigenic variation and  vlsE  recombination, we screened transposon mutants of genes known to be involved in DNA recombination and repair for their effects on infectivity and  vlsE  recombination. Several mutants, including those in BB0023 ( ruvA ), BB0022 ( ruvB ), BB0797 ( mutS ), and BB0098 ( mutS-II ), showed reduced infectivity in immunocompetent C3H/HeN mice. Mutants in  ruvA  and  ruvB  exhibited greatly reduced rates of  vlsE  recombination in C3H/HeN mice, as determined by restriction fragment polymorphism (RFLP) screening and DNA sequence analysis. In severe combined immunodeficiency (C3H/ scid ) mice, the  ruvA  mutant retained full infectivity; however, all recovered clones retained the ‘parental’  vlsE  sequence, consistent with low rates of  vlsE  recombination. These results suggest that the reduced infectivity of  ruvA  and  ruvB  mutants is the result of ineffective  vlsE  recombination and underscores the important role that  vlsE  recombination plays in immune evasion. Based on functional studies in other organisms, the RuvAB complex of  B. burgdorferi  may promote branch migration of Holliday junctions during  vlsE  recombination. Our findings are consistent with those in the accompanying article by Dresser et al., and together these studies provide the first examples of trans-acting factors involved in  vlsE  recombination.

Central Role of the Holliday Junction Helicase RuvAB in vlsE Recombination and Infectivity of Borrelia burgdorferi