Alternate Sigma Factor RpoS Is Required for the In Vivo-Specific Repression ofBorrelia burgdorferiPlasmid lp54-BorneospAandlp6.6Genes

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Abstract
While numerous positively regulated loci have been characterized during the enzootic cycle ofBorrelia burgdorferi, very little is known about the mechanism(s) involved in the repression of borrelial loci either during tick feeding or within the mammalian host. Here, we report that the alternative sigma factor RpoS is required for the in vivo-specific repression of at least two RpoD-dependentB. burgdorferiloci,ospAandlp6.6. The downregulation ofospAandIp6.6appears to require either a repressor molecule whose expression is RpoS dependent or an accessory factor which enables RpoS to directly interact with theospAandIp6.6promoter elements, thereby blocking transcription by RpoD. The central role for RpoS during the earliest stages of host adaptation suggests that tick feeding imparts signals to spirochetes that trigger the RpoS-dependent repression, as well as expression, of in vivo-specific virulence factors critical for the tick-to-mammalian host transition.

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