Protein PII regulates both inorganic carbon and nitrate uptake and is modified by a redox signal in synechocystis PCC 6803.

In Synechocystis PCC 6803 as in other cyanobacteria, involvement of protein PII in the co-regulation of inorganic carbon and nitrogen metabolism was established based on post-translational modifications of the protein resulting from changes in the carbon/nitrogen regimes. Uptake of bicarbonate and nitrate in response to changes of the carbon and/or nitrogen regimes is altered in a PII-null mutant, indicating that both processes are under control of PII. Modulation of electron flow by addition of methyl viologen with or without duroquinol, or in a NAD(P)H dehydrogenase-deficient mutant, affects the phosphorylation level of PII. The redox state of the cells would thus act as a trigger for PII phosphorylation.