Subgenes encoding the lipoyl domains from the acetyltransferase components of two types of pyruvate dehydrogenase complex of Escherichia coli were made by site-specific oligonucleotide-directed nonsense mutagenesis of the corresponding aceF genes. One of the domains is capable of binding lipoic acid whereas the other is not. The subgenes were cloned into an expression vector under the transcriptional control of the lambda PL and lambda PR promoters and a temperature-sensitive lambda repressor. Under non-permissive conditions expression of the lipoyl domains was not detected, but 6 h after thermo-induction the domains were amplified by at least 35-50-fold relative to the normal amounts of each type of covalently bound domain.