The signal transducer PII and bicarbonate acquisition in Prochlorococcus marinus PCC 9511, a marine cyanobacterium naturally deficient in nitrate and nitrite assimilation

The amino acid sequence of the signal transducer P-II (GinB) of the oceanic photosynthetic prokaryote Prochlorococcus marinus strain PCC 9511 displays a typical cyanobacterial signature and is phylogenetically related to all known cyanobacterial glnB genes, but forms a distinct subclade with two other marine cyanobacteria. P-II of A marinus was not phosphorylated under the conditions tested, despite its highly conserved primary amino acid sequence, including the seryl residue at position 49, the site for the phosphorylation of the protein in the cyanobacterium Synechococcus PCC 7942. Moreover, A marinus lacks nitrate and nitrite reductase activities and does not take up nitrate and nitrite. This strain, however, expresses a low- and a high-affinity transport system for inorganic carbon (C-i; K-m,K-app 240 and 4 muM, respectively), a result consistent with the unphosphorylated form of P-II acting as a sensor for the control of C-i acquisition, as proposed for the cyanobacterium Synechocystis PCC 6803. The present data are discussed in relation to the genetic information provided by the A marinus MED4 genome sequence.