UPTAKE OF INORGANIC CARBON IN THE CYANOBACTERIUM SYNECHOCYSTIS PCC6803 - PHYSIOLOGICAL AND GENETIC-EVIDENCE FOR A HIGH-AFFINITY UPTAKE SYSTEM

Synechocystis PCC6803 displays two inorganic carbon-uptake processes, a low-affinity one (apparent K-m:300-400 mu M) functional in cells grown under standard or limiting inorganic carbon concentrations, and one with a higher affinity (60 +/- 12 mu M), detected only in cells adapted to limiting inorganic carbon conditions. A mutational and screening procedure allowed the isolation of a mutant deficient in the high-affinity system, but only slightly impaired in its growth capacities. The mutated genomic region revealed two open reading frames (ORFs), possibly belonging to an operonic structure. A clone in which the downstream ORF, hatR (high-affinity transport), had been inactivated showed a phenotype close to that of the original mutant. Inactivation of the other ORF, hatA, yielded a clone unable to grow in limiting inorganic carbon conditions. The deduced HatA protein showed no homology with any registered protein. It possessed three hydrophobic domains, including a putative signal peptide, Several hypotheses are considered as to its role. The deduced HatR protein, which possessed the features characteristic of the response regulators of the two-component regulatory systems ubiquitous in bacteria, might be a regulator controlling the activity of the high-affinity transport process. It would belong to the subclass of these molecules lacking the DNA-binding domain.