The ggtA gene encodes a subunit of the transport system for the osmoprotective compound glucosylglycerol in Synechocystis sp strain PCC 6803

The ggtA gene was sequenced during the analysis of a mutant of Synechocystis sp. strain PCC 6803 with impaired salt tolerance. It showed striking sequence similarities to ATP-binding proteins of binding-protein-dependent transport systems (ABC transporters). Mutants of ggtA and three neighboring reading frames were constructed by inserting an aphII gene cassette and were physiologically and genetically characterized. The ggtA insertion mutant lost its glucosylglycerol (GG) uptake ability, but its salt tolerance did not change. Therefore, it was concluded that active transport of the osmoprotective compound GG in Synechocystis is mediated by an ABC transporter. The genes for the GG-specific ABC transporter are not organized in an operon as usually found for comparable transporters, since the other insertion mutants showed normal GG transport activity. After cultivation of the ggtA mutant at high salt concentrations, significant amounts of GG were found in the cultivation medium, indicating that GG transport is mainly necessary for recovery of GG leaked through the cytoplasmic membrane. The Northern blot technique revealed increased transcription of the ggtA gene in cells adapted to higher salt concentrations, whereas in cells from basal medium, its transcription was weak.