Abstract
Cyanobacteria harbor unique photoreceptors named cyanobacteriochrome. Many cyanobacteriochromes that bind phycoviolobilin (PVB) as a chromophore exhibit reversible photoconversion between blue and green-absorbing forms (Pb and Pg, respectively). Previous studies suggested that both Z/E isomerization of PVB and adduct formation with a Cys residue take place during Pb/Pg photoconversion. Here, we studied a new member of the Pb/Pg-type cyanobacteriochrome Tlr1999 from Thermosynechococcus elongatus BP-1. The isolated Tlr1999 showed the photoconversion between Pb (418nm) and Pg (498nm). In order to analyze the role of Cys residue, we used iodoacetamide (IAA) that binds irreversibly to thiol groups. When Pg was treated with IAA, there was no great effect, but green light irradiation converted Pg to a new form peaking at 552nm that was inactive in further photoconversion (P552). When Pb was treated, Pb peak was reduced with concomitant generation of P552. Additional treatment with dithiothreitol reverted P552 to a photoactive Pb-like form. These results demonstrated that reversible binding of a thiol group rather than the Cys residue is crucial for reversible Pb/Pg photoconversion.
