A leucine auxotroph of Streptomyces lividans (Sl), designated PC196, was unable to convert alpha-isopropylmalate into the beta-isomer. A DNA fragment from Actinoplanes teichomyceticus (At) cloned into the Streptomyces vector pIJ702 complemented PC196. Sequence analysis of the 3.0-kb insert revealed one complete ORF with high similarity to other leuC genes encoding the large subunit of isopropylmalate isomerase (IPMI), and the 5' end of a second ORF corresponding to leuD, which encodes the smaller subunit of IPMI. Further subcloning established that Sl strain PC196 is defective in the large subunit of IPMI.