CooA is a CO-sensing transcriptional activator from the photosynthetic bacterium Rhodospirillum rubrum that binds CO at the heme iron. The heme iron in ferrous CooA has two axial ligands: His77 and Pro2. CO displaces Pro2 and induces a conformational change in CooA. The dissociation of CO and/or ligation of the Pro2 residue are believed to trigger structural changes in the protein. Visible time-resolved resonance Raman spectra obtained in this study indicated that the nu(Fe-His) mode, arising from the proximal His77 iron stretch, does not shift until 50 after the photodissociation of CO. Ligation of the Pro2 residue to the heme iron was observed around 50 its after the photodissociation of CO, suggesting that the nu(Fe-His) band exhibits no shift until the ligation of Pro2. UV resonance Raman spectra suggested structural changes in the vicinity of Trp110 in the C-helix upon binding, but no or very small spectral changes in the time-resolved UV resonance Raman spectra were observed from 100 ns to 100 its after the photodissociation of CO. These results strongly suggest that the conformational change of CooA is induced by the ligation of Pro2 to the heme iron.