FadR activates the fabH promoter in the absence of long-chain FA [5]. Multiple promoters seem to be a common feature of FA synthesis genes [2]. FadR belongs to the GntR family [24, 25]. However, Xu et al. (2001) reported that homologies to CAP and the Tet repressor based on structure are more relevant and they have categorized FadR as a chimera of two motifs [23]. FadR appears to be a two-domain dimeric molecule in which the N-terminal domains [1, 17, 26, 27] bind to DNA, whereas the C-terminal domain binds the fatty acyl coenzyme A (acyl-CoA) [23]. In addition, there is a linker that connects both the N-terminal and C-terminal. FadR binds DNA as an apo-protein, and it is released when it binds long-chain acyl-CoA [28, 29]. The binding of acyl-CoA disrupts a buried network of charged and polar residues in the C-terminal domain, and the resulting conformational change is transmitted to the N-terminal domain via a domain-spanning α-helix CoA [23]; in this way there is loss of DNA binding [30], since acyl-CoA regulates DNA binding by FadR [1, 31]. A single point mutation of W60G in the FadR regulatory protein from Escherichia coli K113 strain causes loss of its DNA-binding activity and its regulatory roles in lipid metabolism [32]. A hydrophobic interaction among three amino acids (W60, F74, and W75) is critical for its DNA-binding ability by maintaining the configuration of its neighboring two β-sheets [32]. The α/β N-terminal domain (α1-β1-α2-α3-β2-β3) has a winged-helix motif, and the α-helical C-terminal domain (α6-α7-α8-α9-α10-α11-α12) resembles the sensor domain of the Tet repressor [23] and PAS domain, in particular the photoactive yellow protein [30], and finally the linker comprising two short α-helices (α4-α5) [23]. The binding of FadR to DNA is determined by the localization of the α3 recognition helices that are paired together at the dimer interface [23]. The DNA-binding domain is very highly conserved among FadR-containing bacteria, whereas the C-terminal acyl-CoA-binding domain shows only weak conservation [33]. A FadR-type regulator has been identified in Vibrio vulnificus [34], Corynebacterium glutamicum [35], Salmonella enterica, Vibrio cholerae, Pasteurella multocida, and Hemophilus influenzae [33]. FadR is a homodimer [23, 27] that recognizes a palindromic sequence, 5'-TGGNNNNNCCA-3' [23]. Overexpression of FadR improves fatty acid production by 5- to 7.5-fold over that of strains in which it is not overexpressed [6]. Genes related to the stress response proteins and fatty acid biosynthetic pathways are upregulated by FadR [6]. An increase of organic solvent tolerances (OSTs) was shown by using the fadR and marR double mutant of those two transcriptional regulators [36]. L-threonine production increases when the fadR, fabR, and iclR regulators are deleted, suggesting that coupling fatty acid degradation and the L-threonine biosynthesis pathway via the glyoxylate shunt could efficiently increase L-threonine production [37] Reviews: [28, 38]