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International Symposium on Photosynthetic Prokaryotes Meeting Report
ABC transporters in phototrophic prokaryotes
by Gabi Fiedler (U. Regensburg)

Transport systems of the ABC (ATP-Binding Cassette) superfamily facilitate ATP-dependent import and export of a great variety of substrates and are common in bacteria and eukaryotes. Usually the genes encoding these multiprotein complexes are organized in operons.

Victor Bartsevich in Himadri Pakrasi's lab (Washington University, St. Louis) presented his work on the ABC transporter system for manganese in Synechocystis PCC 6803. Deletion of one of the subunits of this Mn2+importer results in reduced growth rates in Mn2+-deficient medium under photoautotrophic conditions due to deficiency in Mn2+-containing PSII. The transporter consists of an ATP-binding protein (MntA), a periplasmic substrate-binding protein (MntC) and an integral membrane protein (MntB) with eight putative transmembrane regions. Two alpha/beta domains were predicted from the sequence, possibly forming a cleft where the substrate binds. A Ca2+-binding loop was also predicted, and Victor discussed the possibility that Ca2+ binding may be involved in stabilization of MntC. In addition to the high affinity ABC transporter that is induced in a Mn2+-deficient medium (< 0.5 µM), a second transport system for Mn2+ also exists, induced by µM levels of Mn2+.

Ana Valladares from Enrique Flores' group (University of Sevilla) presented evidence of a high affinity ammonium transport system and an ABC-type urea importer in Synechocystis PCC 6803, both subject to nitrogen-control mediated by the NtcA transcriptional regulator.

Werner Klipp (Bochum U.) reported that Rhodobacter capsulatus possesses two different molybdenum uptake systems. One of them, the high affinity permease, resembles an ABC transporter consisting of ModA (periplasmic binding protein), ModB (integral membrane protein with five membrane-spanning regions) and ModC (ATP-binding component). Molybdenum binds, if present in high concentrations, to the MopAB proteins, which are DNA-binding proteins and repress the transcription of the modABC genes as well as the genes for a molybdenum-independent nitrogenase.

Glucosylglycerol (GG) is synthesized by the moderately halotolerant Synechocystis PCC 6803 as an osmoprotective compound. Stefan Mikkat (Rostock U.) showed that mutants defective in the genes ggtA, B, C, or D, encoding the subunits of an ABC transporter, are deficient in uptake of [14C]GG. The main function of this ABC importer may be the reuptake of GG that leaks out of cells. Unusual for bacterial ABC transporters is the fact that the gene (ggtA) encoding the ATP-binding subunit is not contiguous with the genes ggtBCD encoding the substrate-binding and the integral membrane protein.

Iris Maldener and Gabriele Fiedler (Regensburg U.) showed that the ABC transporter DevBCA (Development) is essential for maturation of heterocysts of Anabaena 7120. Anabaena strains mutated in devB, C or A are able to synthesize heterocyst-specific glycolipids, but their heterocysts lack the laminated glycolipid layer, suggesting that the DevBCA transporter is involved in export of these specific glycolipids or of an enzyme that is essential for assembly of the laminated layer.