Aspartate aminotransferase (AspAT) was purified to homogeneity from cell extracts of the non-N2-fixing cyanobacterium Phormidium lapideum. The NH2-terminal sequence of 25 amino acid residues was different from the sequences of the subfamily Ialpha of AspATs from eukaryotes and Escherichia coli, but it was similar to sequences of the subfamily Igamma of AspATs from archaebacteria and eubacteria. The enzyme was most active at 80 degrees C and was stable at up to 75 degrees C. Thermal inactivation (60-85 degrees C) of the enzyme followed first-order kinetics, with 2-oxoglutarate causing a shift of the thermal inactivation curves to higher temperatures. However, at 25 degrees C the kcat of P. lapideum AspAT was nearly equal to the values of AspATs from mesophilic organisms. The enzyme used L-aspartate and L-cysteine sulfinate as amino donors and 2-oxoglutarate as an amino acceptor. The Km values were 5.0 mM for L-aspartate, 5.7 mM for L-glutamate, 0.2 mM for 2-oxoglutarate, and 0.032 mM for oxaloacetate.