CLONING, SEQUENCING, AND MOLECULAR ANALYSIS OF THE SOL OPERON OF CLOSTRIDIUM-ACETOBUTYLICUM, A CHROMOSOMAL LOCUS INVOLVED IN SOLVENTOGENESIS

A DNA region of Clostridium acetobutylicum contiguous with the adc ope ron has been cloned and sequenced. Structural genes encoding the aceto acetyl coenzyme A:acetate/butyrate:coenzyme A transferase (ctfB and ct fA) and an alcohol/aldehyde dehydrogenase (adhE) could be identified. These three genes together with a small open reading frame (ORF) of un known function (upstream of adhE) formed an operon (sol operon), as sh own by mRNA analyses. The complete sol operon was transcriptionally in duced or derepressed before the onset of solventogenesis, thus confirm ing earlier results of Northern hybridizations with a ctfB gene probe (U. Gerischer and P. Durre, J. Bacteriol. 174:426-433, 1992). Upstream of the sol operon, we identified two putative promoters that were loc ated in regions with possible stem-loop structures formed by several i nverted repeats. The distal promoter P1 showed only minor transcriptio n initiation in solventogenic C acetobutylicum cells but was recognize d in Escherichia coli, presumably because of its high similarity to th e sigma70 consensus sequence. The adhE-proximal promoter P2 directed t he major transcription start point in solventogenic C acetobutylicum b ut was not recognized in E. coli. The clostridial AdhE showed high sim ilarity to a novel family (type III) of alcohol dehydrogenases. Two ot her ORFs (ORF 5 and ORF 6) were found on the cloned DNA region that sh owed no significant similarity to sequences in various available data bases. mRNA studies revealed that ORF 5 formed a monocistronic operon and showed increased expression before onset of solventogenesis.