Regulation of the sol locus genes for butanol and acetone formation in Clostridium acetobutylicum ATCC 824 by a putative transcriptional repressor

A gene (orf1, now designated solR) previously identified upstream of the al dehyde/alcohol dehydrogenase gene aad (R. V. Nair, G. N. Bennett, and E. T. Papoutsakis, J. Bacteriol, 176:871-885, 1994) was found to encode a repres sor of the sol locus (aad, ctfA, ctfB and adc) genes for butanol and aceton e formation in Clostridium acetobutylicum ATCC 824. Primer extension analys is identified a transcriptional start site 35 bp upstream of the solR start codon, Amino acid comparisons of SolR identified a potential helix-turn-he lix DNA-binding motif in the C-terminal half towards the center of the prot ein, suggesting a regulatory role. Overexpression of SolR in strain ATCC 82 4(pCO1) resulted in a solvent-negative phenotype owing to its deleterious e ffect on the transcription of the sol locus genes. Inactivation of solR in C. acetobutylicum via homologous recombination yielded mutants B and H (ATC C 824 solR::pO1X) which exhibited deregulated solvent production characteri zed by increased flux towards butanol and acetone formation, earlier induct ion of aad, lower overall acid production, markedly improved yields of solv ents on glucose, a prolonged solvent production phase, and increased biomas s accumulation compared to those of the wild-type strain.