Thioesterase I, a constituent domain of the multifunctional fatty acid synthase, and thioesterase II, an independent monofunctional protein, catalyse the chain-terminating reaction in fatty acid synthesis de novo at long and medium chain lengths respectively. The enzymes have been cloned and expressed in Escherichia coli under the control of the temperaturesensitive A repressor. The recombinant proteins are full-length catalytically competent thioesterases with specificities indistinguishable from those of the natural enzymes.

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