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A recent study indicated that Bacillus subtilis catabolizes acetoin by enzymes encoded by the acu gene cluster (F. J. Grundy, D. A. Waters, T. Y. Takova, and T. M. Henkin, Mol. Microbiol. 10:259–271, 1993) that are completely different from those in the multicomponent acetoin dehydrogenase enzyme system (AoDH ES) encoded by aco gene clusters found before in all other bacteria capable of utilizing acetoin as the sole carbon source for growth. By hybridization with a DNA probe covering acoA and acoB of the AoDH ES from Clostridium magnum , genomic fragments from B. subtilis harboring acoA , acoB , acoC , acoL , and acoR homologous genes were identified, and some of them were functionally expressed in E. coli . Furthermore, acoA was inactivated in B. subtilis by disruptive mutagenesis; these mutants were impaired to express PP i -dependent AoDH E1 activity to remove acetoin from the medium and to grow with acetoin as the carbon source. Therefore, acetoin is catabolized in B. subtilis by the same mechanism as all other bacteria investigated so far, leaving the function of the previously described acu genes obscure.

E2 (dihydrolipoamide acetyltransferase) and E3 (dihydrolipoamide dehydrogenase) of the Clostridium magnum acetoin dehydrogenase enzyme system were copurified in a three-step procedure from acetoin-grown cells. The denatured E2-E3 preparation comprised two polypeptides with M(r)s of 49,000 and 67,000, respectively. Microsequencing of both proteins revealed identical amino acid sequences. By use of oligonucleotide probes based on the N-terminal sequences of the alpha and beta subunits of E1 (acetoin dehydrogenase, thymine PPi dependent), which were purified recently (H. Lorenzl, F.B. Oppermann, B. Schmidt, and A. Steinbüchel, Antonie van Leeuwenhoek 63:219-225, 1993), and of E2-E3, structural genes acoA (encoding E1 alpha), acoB (encoding E1 beta), acoC (encoding E2), and acoL (encoding E3) were identified on a single ClaI restriction fragment and expressed in Escherichia coli. The nucleotide sequences of acoA (978 bp), acoB (999 bp), acoC (1,332 bp), and acoL (1,734 bp), as well as those of acoX (996 bp) and acoR (1,956 bp), were determined. The amino acid sequences deduced from acoA, acoB, acoC, and acoL for E1 alpha (M(r), 35,532), E1 beta (M(r), 35,541), E2 (M(r), 48,149), and E3 (M(r), 61,255) exhibited striking similarities to the amino acid sequences of the corresponding components of the Pelobacter carbinolicus acetoin dehydrogenase enzyme system and the Alcaligenes eutrophus acetoin-cleaving system, respectively. Significant homologies to the enzyme components of various 2-oxo acid dehydrogenase complexes were also found, indicating a close relationship between the two enzyme systems. As a result of the partial repetition of the 5' coding region of acoC into the corresponding part of acoL, the E3 component of the C. magnum acetoin dehydrogenase enzyme system contains an N-terminal lipoyl domain, which is unique among dihydrolipoamide dehydrogenases. We found strong similarities between the AcoR and AcoX sequences and the A. eutrophus acoR gene product, which is a regulatory protein required for expression of the A. eutrophus aco genes, and the A. eutrophus acoX gene product, which has an unknown function, respectively. The aco genes of C. magnum are probably organized in one single operon (acoABXCL); acoR maps upstream of this operon.

Dihydrolipoamide dehydrogenase (DHLDH), dihydrolipoamide acetyltransferase (DHLTA), and acetoin: 2,6-dichlorophenolindophenol oxidoreductase (Ao:DCPIP OR) were purified from acetoin-grown cells of Pelobacter carbinolicus. DHLDH had a native Mr of 110,000, consisted of two identical subunits of Mr 54,000, and reacted only with NAD(H) as a coenzyme. The N-terminal amino acid sequence included the flavin adenine dinucleotide-binding site and exhibited a high degree of homology to other DHLDHs. DHLTA had a native Mr of greater than 500,000 and consisted of subunits identical in size (Mr 60,000). The enzyme was highly sensitive to proteolytic attack. During limited tryptic digestion, two major fragments of Mr 32,500 and 25,500 were formed. Ao:DCPIP OR consisted of two different subunits of Mr 37,500 and 38,500 and had a native Mr in the range of 143,000 to 177,000. In vitro in the presence of DCPIP, it catalyzed a thiamine pyrophosphate-dependent oxidative-hydrolytic cleavage of acetoin, methylacetoin, and diacetyl. The combination of purified Ao:DCPIP OR, DHLTA, and DHLDH in the presence of thiamine pyrophosphate and the substrate acetoin or methylacetoin resulted in a coenzyme A-dependent reduction of NAD. In the strictly anaerobic acetoin-utilizing bacteria P. carbinolicus, Pelobacter venetianus, Pelobacter acetylenicus, Pelobacter propionicus, Acetobacterium carbinolicum, and Clostridium magnum, the enzymes Ao:DCPIP OR, DHLTA, and DHLDH were induced during growth on acetoin, whereas they were absent or scarcely present in cells grown on a nonacetoinogenic substrate.

Use of oligonucleotide probes, which were deduced from the N-terminal sequences of the purified enzyme components, identified the structural genes for the alpha and beta subunits of E1 (acetoin:2,6-dichlorophenolindophenol oxidoreductase), E2 (dihydrolipoamide acetyltransferase), and E3 (dihydrolipoamide dehydrogenase) of the Pelobacter carbinolicus acetoin dehydrogenase enzyme system, which were designated acoA, acoB, acoC, and acoL, respectively. The nucleotide sequences of acoA (979 bp), acoB (1,014 bp), acoC (1,353 bp), and acoL (1,413 bp) as well as of acoS (933 bp), which encodes a protein with an M(r) of 34,421 exhibiting 64.7% amino acid identity to the Escherichia coli lipA gene product, were determined. These genes are clustered on a 6.1-kbp region. Heterologous expression of acoA, acoB, acoC, acoL, and acoS in E. coli was demonstrated. The amino acid sequences deduced from acoA, acoB, acoC, and acoL for E1 alpha (M(r), 34,854), E1 beta (M(r), 36,184), E2 (M(r), 47,281), and E3 (M(r), 49,394) exhibited striking similarities to the amino acid sequences of the components of the Alcaligenes eutrophus acetoin-cleaving system. Homologies of up to 48.7% amino acid identity to the primary structures of the enzyme components of various 2-oxo acid dehydrogenase complexes also were found. In addition, the respective genes of the 2-oxo acid dehydrogenase complexes and of the acetoin dehydrogenase enzyme system were organized very similarly, indicating a close relationship of the P. carbinolicus acetoin dehydrogenase enzyme system to 2-oxo acid dehydrogenase complexes.