Antigenic Variation in Occurs Independently of RecQ-Mediated Unwinding of the G Quadruplex.

The obligate human pathogen alters its cell surface antigens to evade the immune system in a process known as antigenic variation (AV). During pilin AV, portions of the expressed pilin gene () are replaced with segments of silent pilin genes () through homologous recombination. The exchange is initiated by formation of a parallel guanine quadruplex (G4) structure near the gene, which recruits the homologous recombination machinery. The RecQ helicase, which has been proposed to aid AV by unwinding the G4 structure, is an important component of this machinery. However, RecQ also promotes homologous recombination through G4-independent duplex DNA unwinding, leaving the relative importance of its G4 unwinding activity unclear. Previous investigations revealed a guanine-specific pocket (GSP) on the surface of RecQ that is required for G4, but not duplex, DNA unwinding. To determine whether RecQ-mediated G4 resolution is required for AV, strains that encode a RecQ GSP variant that cannot unwind G4 DNA were created. In contrast to the hypothesis that G4 unwinding by RecQ is important for AV, the RecQ GSP variant strains had normal AV levels. Analysis of a purified RecQ GSP variant confirmed that it retained duplex DNA unwinding activity but had lost its ability to unwind antiparallel G4 DNA. Interestingly, neither the GSP-deficient RecQ variant nor the wild-type RecQ could unwind the parallel G4 nor the prototypical c- G4. Based on these results, we conclude that AV occurs independently of RecQ-mediated G4 resolution. The pathogenic bacteria avoids clearance by the immune system through antigenic variation (AV), the process by which immunogenic surface features of the bacteria are exchanged for novel variants. RecQ helicase is critical in AV and its role has been proposed to stem from its ability to unwind a DNA secondary structure known as a guanine quadruplex (G4) that is central to AV. In this work, we demonstrate that the role of RecQ in AV is independent of its ability to resolve G4s and that RecQ is incapable of unwinding the G4 in question. We propose a new model of RecQ's role in AV where the G4 might recruit or orient RecQ to facilitate homologous recombination.