Heme-containing proteins, the heme proteins, are known to have physiologic functions in humans, mammalians, fish, plants, and bacteria. For example, hemoglobin and myoglobin, which belong to the globin family, have been studied in terms of their structures and functions with spectroscopic and mutagenic methods. Recently, a new class of heme proteins has been discovered, referred to as gas sensors. These are heme-based sensor proteins that play important roles in transcriptional activation, histidinekinase activities, phosphodiesterase activities, etc. CooA is a CO-sensing transcriptional activator derived from the photosynthetic bacterium Rhodospirillum rubrum. FixL is a rhizobial oxygen sensor protein, and we have targeted Bj FixL derived from Bradyrhizobium japonicum. Dos from Escherichia coli is an oxygen sensor protein, which senses oxygen in the heme-containing domain and induces phosphodiesterase activity in other domains. In previous work, we studied the axial ligands and C-helix of CooA to clarify the activation mechanism. Moreover, FixL and Dos were investigated using time-resolved spectroscopic methods. Whereas FixL has a pentacoordinate heme in the ferrous deoxy form, there are a proximal histidine (His 77) and a distal methionine (Met 95) as axial ligands to coordinate to the heme iron in EcDos. However almost all gas sensors show mono-exponential rebinding (6-7 ps), while EcDosH and full-length Dos show biexponential rebinding (7 ps and 35 ps) on the internal ligand. The results were also supported by molecular dynamic simulation. Here we discuss recent work on gas sensors with implications provided by our research.