Maleate cis-trans isomerase, which catalyses the conversion of maleate to fumarate, was purified and characterized from Alcaligenes faecalis IFO13111. The molecular weight of maleate isomerase was estimated as 60 kDa, consisting of a 28 kDa dimer as shown by gel-filtration chromatography and SDS-PAGE analysis. Kinetic studies showed that the Michaelis constant for maleate was 4.0 x 10(-5) M. The reverse reaction (fumarate to maleate) activity of the enzyme was detected even though it was quite weak. The maleate isomerase gene (maiA) was cloned by hybridization using the oligonucleotide DNA probes designed on the basis of the determined N-terminal amino acid sequences of the purified enzyme. The determined DNA sequence of the maiA gene contains an open reading frame which encodes a 254-amino-acid sequence. The amino acid sequence of the maiA gene product shows no significant homology to any amino acid sequences in the protein data base.