Figure 7. Cross-regulatory network of T4P-like surface structures of S.acidocaldarius.

Operon and structural organization of Aap-pilus, archaellum and Ups-Pilus are shown. Known promoters of the operons are indicated by arrows. Orange arrows indicate pathways that are activated by nutrient limitation. Black arrows = unknown physiological conditions. − = repression, + = activation. P = phosphorylation. All by now identified key players of the archaellum regulatory network (Arn) and their regulatory activity on the Aap-, Ups-pili and archaellum is shown. ArnR is induced by nutrient limitation, and binds to flaB, aapF and upsX promoters. ArnR1 is induced by unknown physiological conditions and binds to aapF and flaB promoters. Both proteins are phosphorylated by ArnC. The membrane-bound sensor kinase ArnS is induced by nutrient limitation, but targets of phosphorylation are so far unknown. AbfR1, a repressor of aapA and activator of flaB is also phosphorylated by ArnC (Orell et al., 2013; Li et al., 2017. ArnA and ArnB are archaellum repressors under nutrient rich conditions, and are phosphorylated by ArnC and ArnD (Reimann et al., 2012). The phosphatase PP2A is involved in regulating the phosphorylation status of ArnA and ArnB (Reimann et al., 2013).