We have inactivated the genes encoding components of MntABC, an ABC (ATP binding cassette) transporter system for manganese in the cyanobacterium Synechocystis sp. PCC 6803. The growth rates of these mutant strains were significantly lower in a manganese-deficient medium and were restored to near normal levels upon addition of micromolar concentrations of Mn2+, indicating the presence of a second transport system for manganese in this organism. 54Mn2+ uptake experiments indicated that the MntABC transporter was induced under manganese starvation conditions, whereas the second transporter system was induced in the presence of micromolar levels of manganese. Both of these systems were nonfunctional at low temperatures and could transport trace levels of 54Mn2+, reflecting high affinity active transport. The initial rates of Mn2+ uptake for cells grown with or without manganese exhibited biphasic saturation kinetics, suggesting that Mn2+ can also be accumulated by a low affinity system in these bacteria. The kinetic parameters for the MntABC transporter system are Km = 1-3 microM and Vmax = 3-8 pmol/min.10(8) cells. Accumulation of manganese by this system was competitively inhibited by Cd2+ (Ki = 4-8 microM), Co2+ and Zn2+ (Ki = 8-15 microM). In contrast, the second high affinity system was highly specific for manganese and was not inhibited by any tested metal ion. We have also demonstrated that in this organism, photosynthetic electron transport is necessary for optimal rates of manganese uptake.