The role of specific amino acid residues in β-ketoadipate succinyl-coenzyme A transferase II from Acinetobacter calcoaceticus was investigated. A 1412 base pair BamiHI-EcoRI fragment carrying the catIJ genes was amplified by polymerase chain reaction and inserted into pUCl9 to generate the plasmid pCATEl9. Escherichia coli DH5α (pCATEl9) carrying only the catlJ genes expressed 3-fold higher enzyme activity than the parent strain. Two mutants were constructed by site directed mutagenesis so that glutamate was replaced by a glutamine at positions Gln155 and Gln193 in the ß subunit of the primary amino acid sequence of the CoA transferase. Both mutants produced transferase that was catalytically active suggesting that Glu155 and Glu193 do not participate directly in catalysis.