Appl. Env. Microbiol., published online 5 April 2013, doi:10.1128/AEM.00011-13

The catabolism of pyrazines—odorous compounds produced by bacteria and plants—is poorly understood. Pathways for the degradation of di- and trisubstituted pyrazines have been proposed, but these related routes consistently include a hydroxylation step that cannot be performed on a tetrasubstituted pyrazine such as tetramethylpyrazine (TTMP). To establish a pathway for these pyrazines, Kutanovas et al. first looked for genes that were induced by TTMP in the TTMP-degrading bacterium Rhodococcus jostii TMP1. MS/MS analysis of the primary upregulated protein led to a locus encoding this protein (named TpdA), along with seven other open reading frames. Sequence analysis suggested that the putative proteins included seven enzymes and one transcriptional regulator with similarity to the LuxR transcription factor family. Expression of TpdABC or TpdAB in a non–TTMP-degrading strain led to the production of oxidatively cleaved intermediates, supporting the authors' predictions that TpdA is a flavin monoxygenase and TpdC is a hydrolase, whereas TpdD, a putative flavin reductase, could apparently be replaced by an endogenous enzyme in the nondegrading host. TpdE was confirmed to be a short-chain reductase, yielding N-(3-hydroxybutan-2-yl)acetamide as a potential substrate for the final uncharacterized enzymes. Although further study will be needed to clarify the specific mechanisms of each enzyme, including a potential role for TpdB in destabilizing the aromatic ring, this enzymatic sequence provides the first validated pathway for pyrazine degradation.