SOLVENTOGENIC ENZYMES OF CLOSTRIDIUM-ACETOBUTYLICUM - CATALYTIC PROPERTIES, GENETIC ORGANIZATION, AND TRANSCRIPTIONAL REGULATION

The enzymes acetoacetate decarboxylase and coenzyme A transferase cata lyse acetone production from acetoacetyl-CoA in Clostridium acetobutyl icum. The ade gene encoding the former enzyme is organized in a monoci stronic operon, while the ctf genes form a common transcription unit w ith the gene (adhE) encoding a probable polyfunctional aldehyde/alcoho l dehydrogenase. This genetic arrangement could reflect physiological requirements at the onset of solventogenesis. In addition to AdhE, two butanol dehydrogenase isozymes and a thiolase are involved in butanol synthesis. RNA analyses showed a sequential order of induction for th e different butanol dehydrogenase genes, indicating an in vivo functio n of BdhI in low level butanol formation. The physiological roles of A dhE and BdhII most likely involve high level butanol formation, with A dhE being responsible for the onset of solventogenesis and BdhII ensur ing continued butanol production. Addition of methyl viologen results in artificially induced butanol synthesis which seems to be mediated b y a still unknown set of enzymes. Although the signal that triggers th e shift to solventogenesis has not yet been elucidated, recent investi gations suggest a possible function of DNA supercoiling as a transcrip tional sensor of the respective environmental stimuli.