Bacteriophytochromes are photochromic histidine kinases using a biliverdinchromophore

Phytochromes comprise a principal family of red/far-red light sensors in pl ants(1). Although phytochromes were thought originally to be confined to ph otosynthetic organisms(2,3), we have recently detected phytochrome-like pro teins in two heterotrophic eubacteria, Deinococcus radiodurans and Pseudomo nas aeruginosa(4). Here we show that these form part of a widespread family of bacteriophytochromes (BphPs) with homology to two-component sensor hist idine kinases. Whereas plant phytochromes use phytochromobilin as the chrom ophore, BphPs assemble with biliverdin, an immediate breakdown product of h aem, to generate photochromic kinases that are modulated by red and far-red light. In some cases, a unique haem oxygenase responsible for the synthesi s of biliverdin is part of the BphP operon. Co-expression of this oxygenase with a BphP apoprotein and a haem source is sufficient to assemble holo-Bp hP in vivo. Both their presence in many diverse bacteria and their simplifi ed assembly with biliverdin suggest that BphPs are the progenitors of phyto chrome-type photoreceptors.