FIG. 9.

Common strategy in regulating amino acid transporter genes and biosynthetic genes. Both amino acid transporters and biosynthetic enzymes can fulfill the need of an organism for certain amino acids. This results in a tendency to coordinate the expression of the genes for these classes of proteins by using a shared regulatory mechanism. As shown for three amino acid-related genes, this tendency is specific for each phylogenetic clade. The regulatory elements were identified using our Riboswitch Web server (RibEx) (1) and previously reported data (2). [1], the S_MK riboswitch regulates metK genes in lactic acid bacteria, including Enterococcus, Streptococcus, and Lactococcus spp. (see reference 33). [2], in streptococci, unlike other members of the Firmicutes, methionine biosynthesis and transport are controlled by protein transcription factors, in this case, MtaR, MetR, and CmbR (see reference 67). Organism names and gene color codes are described in the legends of Fig. 2 and 3.