Two intermediate states I and J trapped at low temperature in the photocycles of two BLUF domain proteins of cyanobacteria Synechocystis sp. PCC6803 and Thermosynechococcus elongatus BP-1

We identified the two intermediate states, I and J, that are common in the photocycles of the cyanobacterial BLUF (sensor of Blue Light Using Flavin) domain proteins of Slr1694 of Synechocystis sp. PCC6803 and Tll0078 of Thermosynechococcus elongatus BP-1 by analyzing the absorption spectra at 5 K. Illumination at 5 K accumulated intermediate forms (designated as I5 and I9), which showed 5 and 9 nm redshifts of the absorption bands of flavin in the Tll0078 and Slr1694 proteins, respectively. I5 (I9) was converted into the next intermediate, which have 11 nm (14 nm) red-shifted absorption bands J11 (J14) after dark annealing at 230 K (240 K). Further dark annealing at 280 K (270 K) of J11 (J14) produced the signal-transmitting final form F490 (F495), with a small increase in the absorption at around 490 nm (495 nm). The results indicate that the BLUF proteins of Tll0078 and Slr1694 exhibit the common photocycle of D471 (D467) --> I5 (I9) --> J11 (J14) --> F490 (F495) at low temperature. The transition temperatures for these intermediate forms differ for two proteins. The amount of I5 (I9) accumulated at 5 K was small and increased at a higher temperature, suggesting heterogeneity of the protein structure that determines the reaction pathway.