Characterization of the substrate-binding PotD subunit in Synechocystis sp. strain PCC 6803

The potD gene encodes the bacterial substrate-binding subunit of the polyamine transport system. The uptake system, which belongs to the ABC transporters, has been characterized in Escherichia coli, but it has not been previously studied in cyanobacteria. Although the overall sequence identity between Synechocystis sp. strain PCC 6803 (hereafter Synechocystis) PotD and Escherichia coli PotD is 24%, the ligand-binding site in the constructed homology model of Synechocystis PotD is well conserved. The conservation of the five polyamine-binding residues (Asp206, Glu209, Trp267, Trp293, and Asp295 in Synechocystis PotD) between these two species indicated polyamine-binding capacity for Synechocystis PotD. The Synechocystis potD gene is functional and its expression is under environmental regulation at transcriptional as well as post-transcriptional levels. Furthermore, an in vitro binding assay with the purified recombinant PotD protein demonstrated that the Synechocystis PotD protein is able to bind polyamines and favors spermidine over putrescine. Finally, we confirmed that Synechocystis PotD plays a physiological role in the uptake of polyamines in vivo using a constructed Synechocystis potD-disruption mutant.