Branched-chain keto acid dehydrogenase is a multienzyme complex which is required for the metabolism of the branched-chain amino acids in Pseudomonas putida. The structural genes encoding all four proteins of the bkd operon have been cloned, and their nucleotide sequences have been determined (G. Burns, K. T. Madhusudhan, K. Hatter, and J. R. Sokatch, p. 177-184 in S. Silver, A. M. Chakrabarty, B. Iglewski, and S. Kaplan [ed.], Pseudomonas: Biotransformations, Pathogenesis, and Evolving Biotechnology, American Society for Microbiology, Washington D.C., 1990). An open reading frame which encoded a protein with 36.5% amino acid identity to the leucine-responsive regulatory protein (Lrp) of Escherichia coli was found immediately upstream of the bkd operon. Chromosomal mutations affecting this gene, named bkdR, resulted in a loss of ability to use branched-chain amino acids as carbon and energy sources and failure to produce branched-chain keto acid dehydrogenase. These mutations were complemented in trans by plasmids which contained intact bkdR. Mutations affecting bkdR did not have any effect on transport of branched-chain amino acids or transamination. Therefore, the bkdR gene product must affect expression of the bkd operon and regulation must be positive. Mutations affecting bkdR could also be complemented by plasmids containing lrp of E. coli. This is the first instance of a Lrp-like protein demonstrated to regulate expression of an operon outside of E. coli.