The amino acid substitutions in M2 and M3 were K52N and G141S, respectively. Residue K52 was reported to have a negative effect on the transcription activation of class II CRP-dependent promoters ( Rhodius and Busby, 2000), whereas substitution of the residue with a neutral or negatively charged amino acid (K52N, K52D, or K52L) would improve the binding of CRP to a CRP-dependent promoter