In Archaea and Bacteria， gene expression is tightly regulated in response to environmental stimuli. In the thermoacidophilic crenarchaeon s nutrient limitation induces expression of the archaellum， the archaeal motility structure. This expression is orchestrated by a complex hierarchical network of positive and negative regulators-the archaellum regulatory network (arn). The membrane-bound one-component system ArnR and its paralog ArnR1 were recently described as main activators of archaellum expression in . They regulate gene expression of the archaellum operon by targeting the promoter of ， encoding the archaellum filament protein. Here we describe a strategy for the isolation and biochemical characterization of these two archaellum regulators. Both regulators are capable of forming oligomers and are phosphorylated by the Ser/Thr kinase ArnC. Apart from binding to ， ArnR but not ArnR1 bound to promoter sequences of and ， which encode components of the archaeal adhesive pilus and UV-inducible pili system， demonstrating a regulatory connection between different surface appendages of .