In this research, various mutants of two gas sensor proteins, DOS and CooA, were prepared in order to reveal signal transduction mechanism triggered by gas molecules such as oxygen and CO. Firstly, axial ligands of Dos were identified since this protein was found to contain six-coordinate low-spin heme. On the basis of structural information of FixL which is homologous to Dos, Met 95 of Dos was replaced by Gln, and absorption and resonance Raman spectra were measured. This mutant was found to contain five-coordinate high-spin heme, and the Met95 was established to be one of the axial ligand of Dos. On the other hand, mutation was introduced to amino acid residues in close proximity of the heme cofactor in order to reveal the DNA binding mechanism of CooA, which is a transcription factor and senses CO partial pressure. His77 residue was replaced by Asn, Ser, and Arg which have different polarity, and identity of the axial ligand replaced by CO was investigated. These mutations were found to result in the formation of five-coordinate heme, but bound v(Fe-CO) stretching Raman band was insensitive to the mutations. Thus, CO was suggested to replace Pro2 residue trans to the His77 ligand. In addition, resonance Raman spectra were measured for the mutants Cys105, Leu112, Ile113, Ala114, Leu116, Gly117, Arg118, Leu120, Tnr121, Arg125, and Met131, which reside on the center helix of CooA and are supposed to concern in the signal transduction. The v(Fe-CO) stretching fiequency were found to shift greatly for L116Q, G117N, and L120Q, and these residues were suggested to locate closely to the bound CO gas. Furthermore, footprint analysis revealed that DNA binding activity was lost in these mutants.