S. For instance, the eukaryotic mitogen-activated protein (MAP) cascade is an illustrious example of a tricyclic phosphorylation cascade (619). With respect to ammonium metabolism in E. coli, covalent modification is known to play a prime role in its regulation. Adenylylation of GS, uridylylation of GlnB and of GlnK, and phosphorylation of NRI and of NRII are the three relevant covalent modificati