f Sulfuric volatile compounds derived from cysteine and methionine provide many dairy products with a characteristic odor and taste. To better understand and control the environmental dependencies of sulfuric volatile compound formation by the dairy starter bacteria, we have used the available genome sequence and experimental information to systematically evaluate the presence of the key enzymes and to reconstruct the general modes of transcription regulation for the corresponding genes. The genomic organization of the key genes is suggestive of a subdivision of the reaction network into five modules, where we observed distinct differences in the modular composition between the families Lactobacillaceae, Enterococcaceae, and Leuconostocaceae, on the one hand, and the family Streptococcaceae, on the other. These differences are mirrored by the way in which transcription regulation of the genes is structured in these families. In the Lactobacillaceae, Enterococcaceae, and Leuconostocaceae, the main shared mode of transcription regulation is methionine (Met) T-box-mediated regulation. In addition, the gene metK,
Many of the characteristic flavors in fermented dairy products such as cheese and yoghurt are the result of metabolic reactions involving sulfur-containing amino acids. The microorganisms applied in these products degrade cysteine and methionine, resulting in the production of flavor components such as methanethiol, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS). Insight into the regulatory signals and pathways that control the corresponding metabolic fluxes involved in the formation of these flavor compounds and their precursors is essential to rationally control and steer the flavor profiles of said dairy products.The microorganisms used to produce fermented dairy products belong to the taxonomic order Lactobacillales, which includes the families Enterococcaceae, Lactobacillaceae, Leuconostocaceae, and Streptococcaceae. Many of the respective species are characterized by the fact that they produce lactic acid and are therefore known as the lactic acid bacteria (LAB). The transcription of genes encoding the proteins that are involved in cysteine and methionine metabolism in lactic acid bacteria and other Lactobacillales is controlled by both regulator-binding and RNA structural switches. In various Streptococcaceae, the LysR-family transcription regulators MtaR and CmbR have been shown to be involved in activation as well as repression of genes such as cysD, cysK, metA, metC, metE, and metF (e.g., for Lactococcus lactis [25,70] and Streptococcus mutans [36,68]). The transcription regulator HomR was reported to control the expression of metB in S. mutans and Streptococcus thermophilus (69). In addition, three types of RNA structural switches for the regulation of cysteine and methionine metabolism have been reported for low-GC-content Grampositive bacteria: the T box, the S box, and the S MK box (14,22,59,77,80). These sequence elements at the 5= untranslated region of an...