Phosphorylation-independent activation of the atypical response regulator NblR

Cyanobacteria respond to environmental stress conditions by adjusting their photosynthesis machinery. In Synechococcus sp. PCC 7942, phycobilisome degradation and other acclimation responses after nutrient or high-light stress require activation by the orphan response regulator NbIR, a member of the OmpR/PhoB family. Although NbIR contains a putative phosphorylatable residue (Asp57), it lacks other conserved residues required to chelate the Mg2+ necessary for aspartic acid phosphorylation or to transduce the phosphorylation signal. In close agreement with these features, NbIR was not phosphorylated in vitro by the low-molecular-mass phosphate donor acetyl phosphate and mutation of Asp57 to Ala had no impact on previously characterized NbIR functions in Synechococcus. On the other hand, in vitro and in vivo assays show that the default state of NbIR is monomeric, suggesting that, despite input differences, NbIR activation could involve the same general mechanism of activation by dimerization present in known members of the OmpR/PhoB family. Structural and functional data indicate that the receiver domain of NbIR shares similarities with other phosphorylation-independent response regulators such as FrzS and HP1043. To acknowledge the peculiarities of these atypical 'two-component' regulators with phosphorylation-independent signal transduction mechanisms, we propose the term PIARR, standing for phosphorylation-independent activation of response regulator.