Bathy Phytochromes in Rhizobial Soil Bacteria

Phytochromes are biliprotein photoreceptors that are found in plants, bacteria, and fungi. Prototypical phytochromes have a Pr ground state that absorbs in the red spectral range and is converted by light into the Pfr form, which absorbs longer-wavelength, far-red light. Recently, some bacterial phytochromes have been described that undergo dark conversion of Pr to Pfr and thus have a Pfr ground state. We show here that such so-called bathy phytochromes are widely distributed among bacteria that belong to the orderRhizobiales . We measuredin vivo spectral properties and the direction of dark conversion for species which have either one or two phytochrome genes.Agrobacterium tumefaciens C58 contains one bathy phytochrome and a second phytochrome which undergoes dark conversion of Pfr to Prin vivo . The related speciesAgrobacterium vitis S4 contains also one bathy phytochrome and another phytochrome with novel spectral properties.Rhizobium leguminosarum 3841,Rhizobium etli CIAT652, andAzorhizobium caulinodans ORS571 contain a single phytochrome of the bathy type, whereasXanthobacter autotrophicus Py2 contains a single phytochrome with dark conversion of Pfr to Pr. We propose that bathy phytochromes are adaptations to the light regime in the soil. Most bacterial phytochromes are light-regulated histidine kinases, some of which have a C-terminal response regulator subunit on the same protein. According to our phylogenetic studies, the group of phytochromes with this domain arrangement has evolved from a bathy phytochrome progenitor.