Abstract
BLUF is a novel flavin-binding fold involved in several blue-light input processes. To elucidate the mechanism of its photocycle, we characterize a BLUF-containing protein, Slr1694, from a cyanobacterium Synechocystis sp. PCC 6803. Purified Slr1694 shows a photocycle with light-induced red-shifted absorption change as previously reported in another BLUF-related protein AppA, suggesting that the same initial photochemistry is conserved in all other BLUF-containing proteins. FTIR spectroscopy with 15N- or 13C-labeled, and unlabeled Slr1694 proteins was applied to examine the light-induced structure change of the bound chromophore. Two major bands were assigned to be the weaken force constant change in the flavin C(4)=O and C(2)=O stretching modes, suggesting light-induced formation of hydrogen bonds between amino acids and the flavin C=O groups. We propose that the light-induced hydrogen-bond arrangement couples with the switching process to let the protein in the signalling state with showing red-shifted absorption changes.
