The one‐component system ArnR : a membrane‐bound activator of the crenarchaeal archaellum

Summary Linking the motility apparatus to signal transduction systems enables microbes to precisely control their swimming behaviour according to environmental conditions. Bacteria have therefore evolved a complex chemotaxis machinery, which has presumably spread through lateral gene transfer into the euryarchaeal subkingdom. By contrast C renarchaeota encode no chemotaxis‐like proteins but are nevertheless able to connect external stimuli to archaellar derived motility. This raises fundamental questions about the underlying regulatory mechanisms. Recently, we reported that the thermoacidophilic crenarchaeon S ulfolobus acidocaldarius becomes motile upon nutrient starvation by promoting transcription of flaB encoding the filament forming subunits. Here we describe two transcriptional activators as paralogous one‐component‐systems Saci _1180 and Saci _1171 ( ArnR and ArnR 1). Deletions of arnR and arnR1 resulted in diminished flaB expression and accordingly the deletion mutants revealed impaired swimming motility. We further identified two inverted repeat sequences located upstream of the flaB core promoter of S . acidocaldarius . These cis ‐regulatory elements were shown to be critical for ArnR and ArnR 1 mediated flaB gene expression in vivo . Finally, bioinformatic analysis revealed ArnR to be conserved not only in S ulfolobales but also in the crenarchaeal order of D esulfurococcales and thus might represent a more general control mechanism of archaeal motility.