Role of BkdR, a transcriptional activator of the SigL-dependent isoleucineand valine degradation pathway in Bacillus subtilis

A new gene, bkdR (formerly called yqiX), encoding a regulator with a centra l (catalytic) domain was found in Bacillus subtilis, This gene controls the utilization of isoleucine and valine as sole nitrogen sources. Seven genes , previously called yqiS, yqiT, yqiU, yqiV, bfmBAA, bfnzBAB, and bfmBB and now referred to as ptb, bcd, buk, lpd, bkdA1, bkdA2, and bkdB, are located downstream from the bkdR gene in B. subtilis. The products of these genes a re similar to phosphate butyryl coenzyme A transferase, leucine dehydrogena se, butyrate kinase, and four components of the branched chain keto acid de hydrogenase complex: E3 (dihydrolipoamide dehydrogenase), E1 alpha (dehydro genase), E1 beta (decarboxylase), and E2 (dihydrolipoamide acyltransferase) , Isoleucine and valine utilization was abolished in bcd and bkdR null muta nts of B, subtilis. The seven genes appear to be organized as an operon, bk d, transcribed from a -12, -24 promoter, The expression of the bkd operon w as induced by the presence of isoleucine or valine in the growth medium and depended upon the presence of the sigma factor SigL, a member of the sigma 54 family. Transcription of this operon was abolished in strains containin g a null mutation in the regulatory gene bkdR, Deletion analysis showed tha t upstream activating sequences are involved in the expression of the bkd o peron and are probably the target of bkdR, Transcription of the bkd operon is also negatively controlled by CodY, a global regulator of gene expressio n in response to nutritional conditions.