Fig. 8.

Model of the involvement of phosphorylation in the regulation of motility in S. acidocaldarius. During exponential growth (A) archaellum synthesis from the flaB-flaJ (saci1178–1172) operon is repressed. Part of the repression network are ArnA (Saci1210) and ArnB (Saci1211) that are phosphorylated and dephosphorylated by ePK (Saci1193) and PP2A, respectively. ArnA and B closely interact and repress the flaX promoter (35). Probably another, yet unknown, factor is repressing the flaB promoter. When S. acidocaldarius cells enter stationary phase and undergo starvation, or in the Δsaci_pp2a deletion mutant (B), the ArnaA/B complex exists only in its phosphorylated form and repression is released. Moreover, the expression of ArnR (Saci1180) and ArnR1 (Saci1171) is induced and these factors then drive high expression of FlaB which leads to the assembly of archaella on the cell surface and therefore to increased motility.