Malic enzyme (ME) was purified as an electrophoretically homogenous protein from Rhodopseudomonas palustris No. 7. The molecular weight of ME was estimated to be 650 kDa and that of its subunit, 86 kDa. ME activity was remarkably enhanced by di- and mono-valent cations, and the K(a) values for Mg(2+) and NH(4)(+) were 0.26 and 0.56 mM respectively. Purified ME used both NAD(+) and NADP(+) as electron acceptors, with K(m) values of 0.11 and 1.8 mM. The K(m) value for L-malate was 1.7 mM using NAD(+) as electron acceptor. Gene cloning of the ME indicated that the ME from R. palustris strain No. 7 was composed of 774 amino acids encompassing the ME and phosphotransacetylase domains, although purified ME displayed no phosphotransacetylase activity. ME activity was inhibited by acetyl-CoA, oxaloacetate, and fructose-6-phosphate. These results suggest that ME plays an important role in the metabolic regulation of R. palustris No. 7 under photoheterotrophic conditions.