Regulation of (p)ppGpp hydrolysis by a conserved archetypal regulatory domain.

Sensory and regulatory domains allow bacteria to adequately respond to environmental changes. The regulatory ACT domains are mainly found in metabolic-related proteins as well as in long (p)ppGpp synthetase/hydrolase (SD/HD) enzymes. Here, we investigate the functional role of the ACT domain of SpoT, the only (p)ppGpp SD/HD of Caulobacter crescentus . We show that SpoT requires the ACT domain to hydrolyse ppGpp in an efficient way. In addition, our in vivo and in vitro data show that the phosphorylated version of EIIA Ntr (EIIA Ntr ~P) interacts directly with the ACT to inhibit the hydrolase activity of SpoT. Finally, we highlight the conservation of the ACT-dependent interaction between EIIA Ntr ~P and SpoT/Rel along with the PTS Ntr -dependent regulation of (p)ppGpp accumulation upon nitrogen starvation in Sinorhizobium meliloti , a plant-associated α-proteobacterium. Thus, this work suggests that α-proteobacteria might have inherited from a common ancestor, a PTS Ntr dedicated to modulate (p)ppGpp levels.