“…Light induction of HKs has been reported for several systems, amongst which the first identified cyanobacterial phytochrome, Cph1 (3), for bacteriophytochromes (7,8), and even for blue light-sensitive photoreceptors, where a flavin chromophore-binding domain activates an HK just in the same way as in the red light-sensing phytochromes (9). Recently, we have demonstrated light-inducible kinase activity for two cyanobacterial phytochromes, CphA and CphB from Tolypothrix PCC7601, and the transfer of the phosphate group from the histidine in the HK domain to an added response regulator (10,11). These two proteins show a very similar overall architecture (as deduced from sequence comparison, and secondary and tertiary structure prediction); however, they * This work was supported in part by the Helmholtz-Society (VIBS VH-VI-157, to M. E. and W. G.) and the International Max Planck Research School Chemical Biology (to Y. J. K.).…”