Photoreceptor which exists in two forms that are reversibly interconvertible by light: far-red light (733 nm) converts protein to the red-absorbing (Pr) form, while red light (630 nm) partly converts the protein to the far-red-absorbing (Pfr) form (PubMed:27107635). Regulates virulence of X.campestris pv. campestris on its host plants, perhaps by fine-tuning expression to ambient light levels and/or spatial cues (PubMed:27621284). The Pr form may sense light and partially inhibit virulence; in the dark (Pfr form) biofilm and xanathan production rise and bacteria are more virulent (PubMed:27621284). Strains overexpressing this protein have significantly decreased amounts of extracellular beta-1,4-endoglucanase, produce less xanthin and have decreased transcription of genes involved in virulence such as endoglucanases, type 2 secretion systems, xanthan production and flagellar-dependent motility (PubMed:27621284). Forms head-to-head homodimers (PubMed:27107635). Expressed in dark and light (at protein level). Part of the bphO-bphP operon (PubMed:27621284). Composed of 4 domains; in the N-terminus the PAS2-GAF-PHY domains form the photosensor while the C-terminal PAS9 domain is the output module. Long linker helices serve as connectors within the GAF-PHY and PHY-PAS9 domain pairs, forming a helical spine where the dimer interface is assembled. Thus both the photosensor and PAS9 domains are required for dimerization. Within the PHY domain is the mobile tongue domain (residues 452-480) that probably serves as a gatekeeper to the chromophore-binding pocket, in the Pr form it is a beta-sheet, in a R.palustris Pfr crystal it is an alpha helix (AC B3Q7C0). In the dark (Pfr) state the protein is compact and protease resistant, far-red light treated protein (Pr) is quickly degraded to its domains, with important cuts occurring in the hinge region (just before the PAS9 output domain at about residue 512) and in the tongue domain (about residue 469). Contains one covalently linked biliverdin IX-alpha chromophore; present in the crystal structure as a mixture of Pr and Meta-R configurations. No growth phenotype in liquid culture, slightly better growth on plates, strain loses slight sensitivity to white light. In planta is more virulent, with increased growth in host (A.thaliana) leaves, mutant opens leaf stomata better than wild-type, has decreased leaf callose deposits, produces more xanthan than wild-type and is insensitive to white light, has considerably increased sliding and slightly increased swimming mobility, generates mature biofilm in the light and dark. X.campestris pv. campestris colonizes plant leaves and is responsible for black rot in cruciferous plants (Brassicaceae). In the cruciferous model host organism A.thaliana, morning and midday infections result in more hypersensitive responses than evening or night infections, suggesting light plays a role in virulence. In the N-terminal section; belongs to the phytochrome family.