The photocycle intermediates of photoactive yellow protein (PYP) were characterized by low-temperature Fourier transform infrared spectroscopy. The difference FTIR spectra of PYP(B), PYP(H), PYP(L), and PYP(M) minus PYP were measured under the irradiation condition determined by UV-visible spectroscopy. Although the chromophore bands of PYP(B) were weak, intense sharp bands complementary to the 1163-cm(-1) band of PYP, which show the chromophore is deprotonated, were observed at 1168-1169 cm(-1) for PYP(H) and PYP(L), indicating that the proton at Glu46 is not transferred before formation of PYP(M). Free trans-p-coumaric acid had a 1294-cm(-1) band, which was shifted to 1288 cm(-1) in the cis form. All the difference FTIR spectra obtained had the pair of bands corresponding to them, indicating that all the intermediates have the chromophore in the cis configuration. The characteristic vibrational modes at 1020-960 cm(-1) distinguished the intermediates. Because these modes were shifted by deuterium-labeling at the ethylene bond of the chromophore while labeling at the phenol part had no effect, they were attributed to the ethylene bond region. Hence, structural differences among the intermediates are present in this region. Bands at about 1730 cm(-1), which show that Glu46 is protonated, were observed for all intermediates except for PYP(M). Because the frequency of this mode was constant in PYP(B), PYP(H), and PYP(L), the environment of Glu46 is conserved in these intermediates. The photocycle of PYP would therefore proceed by changing the structure of the twisted ethylene bond of the chromophore.

Photoactive yellow protein (PYP) belongs to the novel group of eubacterial photoreceptor proteins. To fully understand its light signal transduction mechanisms, elucidation of the intramolecular pathway of the internal proton is indispensable because it closely correlates with the changes in the hydrogen-bonding network, which is likely to induce the conformational changes. For this purpose, the vibrational modes of PYP and its photoproduct were studied by Fourier transform infrared spectroscopy at ؊40°C. The vibrational modes characteristic for the anionic p-coumaryl chromophore (Kim, M., Mathies, R. A., Hoff, W. D., and Hellingwerf, K. J. (1995) Biochemistry 34, 12669 -12672) were observed at 1482, 1437, and 1163 cm ؊1 for PYP. However, the bands corresponding to these modes were not observed for PYP M , the blue-shifted intermediate, but the 1175 cm ؊1 band characteristic of the neutral p-coumaryl chromophore was observed, indicating that the phenolic oxygen of the chromophore is protonated in PYP M . A 1736 cm ؊1 band was observed for PYP, but the corresponding band for PYP M was not. Because it disappeared in the Glu-46 3 Gln mutant of PYP, this band was assigned to the C‫؍‬O stretching mode of the COOH group of Glu-46. These results strongly suggest that the proton at Glu-46 is transferred to the chromophore during the photoconversion from PYP to PYP M .Photoactive yellow protein (PYP) 1 ( max ϭ 446 nm) (1) is proposed to be a photoreceptor protein for the negative phototaxis observed in the purple phototrophic bacterium, Ectothiorhodospira halophila (2). PYP belongs to the novel group of photoreceptor proteins (3, 4) whose structures are quite different from those of the other photoreceptor proteins studied so far. Namely, the protein moiety of PYP has an ␣/␤ fold structure (5) composed of 125 amino acids (6, 7). The chromophore is a p-coumaric acid (7-9) bound to a cysteine residue via a thioester bond.PYP absorbs a photon and enters the photocycle. We have analyzed the photocycle of PYP in detail by low temperature spectroscopy and identified several intermediates (10). Irradiation of PYP at Ϫ190°C yields PYP B ( max ϭ 489 nm) and PYP H ( max ϭ 442 nm), which are thermally converted to PYP L ( max ϭ 456 nm) through PYP BL ( max ϭ 400 nm) and PYP HL ( max ϭ 447 nm), respectively. The two pathways beginning with PYP B and PYP H join at PYP L and revert to PYP. Flash photolysis at ambient temperature identified two intermediates, pR ( max ϭ 465 nm) and pB ( max ϭ 355 nm) (11,12). pR is formed within 10 ns after flash excitation. It decays to pB over a submillisecond time scale and reverts to PYP within 1 s. It has been demonstrated that pR is the same species as PYP L (10) (In this paper, pR and pB are called PYP L and PYP M , respectively, to avoid confusion) and that PYP L is accumulated by irradiation of PYP at Ϫ80°C (10, 13). However, the precursors of PYP L have not been discovered by flash photolysis at room temperature.Recent studies have clarified some details of the events that take place during t...