Gac is a transcriptional repressor of the Lyme disease spirochete’s OspC virulence-associated surface protein

The OspC outer-surface lipoprotein is essential for the Lyme disease spirochete’s initial phase of vertebrate infection. Bacteria within the midguts of unfed ticks do not produce OspC, but produce high levels when ticks begin to ingest blood. Lyme disease spirochetes cease production of OspC within 1-2 weeks of vertebrate infection, and bacteria that fail to downregulate OspC are cleared by host antibodies. Thus, tight regulation of OspC levels is critical for survival of Lyme borreliae, and therefore an attractive target for development of novel treatment strategies. Previous studies determined that a DNA region 5’ of the ospC promoter, the ospC operator, is required for control of OspC production, but the mechanism was not determined. Hypothesizing that the ospC operator may bind a regulatory factor, DNA affinity pulldown was performed, and identified binding by the Gac protein. Gac is encoded by the C-terminal half of the gyrA open reading frame, from an internal promoter, ribosome-binding site, and initiation codon. Our analyses determined that Gac exhibits a substantially greater affinity for ospC operator and promoter DNAs than for other tested borrelial sequences. In vitro and in vivo analyses demonstrated that Gac is a transcriptional repressor of ospC. These results constitute a substantial advance to our understanding the mechanisms by which the Lyme disease spirochete controls production of OspC. Importance Borrelia burgdorferi (sensu lato) requires its surface-exposed OspC protein in order to establish infection of humans and other vertebrate hosts. Lyme disease spirochetes initiate production of OspC when the vector tick begins to feed on the blood of a vertebrate host, and the spirochetes ceases production of OspC soon after establishment of infection. Bacteria that either do not produce OspC during transmission, or fail to repress OspC after infection is established, are rapidly cleared by the host. The molecular mechanisms controlling OspC production are largely unstudied. Herein, we identified a borrelial protein, Gac, that binds with high affinity to the ospC promoter and 5’ adjacent DNA. A combination of biochemical analyses and investigations of genetically-manipulated bacteria demonstrated that Gac is a transcriptional repressor of ospC. This is a substantial advance toward understanding how the Lyme disease spirochete controls production of the essential OspC virulence factor, and identifies a novel target for preventative and curative therapies.