Figure 8

Model of signal recognition and activation of the ColRS system. When Zn²⁺ or Fe³⁺ concentration is low, metal ions are not bound by the periplasmic domain of ColS and ColR is not phosphorylated. When P. putida experiences metal excess, a Zn²⁺ or Fe³⁺ ion binds with four glutamic acids of two ExxE motifs from two ColS proteins. Ion binding changes ColS conformation and the conserved histidine (H) in the dimerization and histidine phosphotransfer domain (DHp) is autophosporylated by the catalytic domain (CA) of ColS. Both in cis and in trans phosphorylation mechanisms are presented. Phosphate group is subsequently transferred from ColS to ColR and as a result ColR becomes active as a transcription regulator.