The histidine kinase-related domain participates in phytochrome B function but is dispensable

Abstract

Phytochromes are photoreceptors that control many plant light responses. Phytochromes have two carboxyl-terminal structural domains called the PAS repeat domain and the histidine kinaserelated domain. These domains are each related to bacterial histidine kinase domains, and biochemical studies suggest that phytochromes are light-regulated kinases. The PAS repeat domain is important for proper phytochrome function and can interact with putative signaling partners. We have characterized several new phytochrome B mutants in Arabidopsis that express phyB protein, three of which affect the histidine kinase-related domain. Point mutations in the histidine kinase-related domain cause phenotypes similar to those of null mutants, indicating that this domain is important for phyB signaling. However, a truncation that removes most of the histidine kinase-related domain results in a phyB molecule with partial activity, suggesting that this domain is dispensable. These results suggest that phytochromes evolved in modular fashion. We discuss possible functions of the histidine kinase-related domain in phytochrome signaling. P hytochromes are a family of red͞far-red light photoreceptors in plants that regulate many developmental and cellular responses to light. They are present in all plant species examined and in many algae. The model plant Arabidopsis thaliana has five phytochromes called phyA-phyE that diverge from each other by as much as 50%, and genetic and physiological studies have revealed that they regulate distinct light responses (1-3). Phytochrome proteins interconvert between two stable spectral forms. They are synthesized in the dark in the Pr (red-absorbing) form. Red light converts Pr to the Pfr (far-red-absorbing) form, and far-red light reconverts Pfr to Pr. Pfr (and, for phyA, possibly ''cycled'' Pr that has passed through the Pfr form) is thought to be the biologically active form of phytochrome (ref. 4 and discussed in ref. 5).Phytochromes have two major structural domains (6, 7). The amino-terminal domain (Ϸ74 kDa) has a covalently attached linear tetrapyrrole chromophore (phytochromobilin) and is sufficient for light absorption and photoreversibility. The carboxylterminal domain (Ϸ55 kDa) is important for dimerization and downstream signaling and consists of two subdomains termed the PAS repeat domain (PRD) and the histidine kinase-related domain (HKRD). These domains are 9%-11% identical to each other within representative phytochromes, and they are each 13%-17% identical to the histidine kinase domain of sensory transducers of bacterial two-component regulatory systems (8).The cyanobacterium Synechocystis has a phytochrome-like molecule called cph1, which has an amino-terminal chromophore domain very similar to that of higher plant phytochromes and a single carboxyl-terminal histidine kinase domain. Like other two-component sensory transducers, cph1 autophosphorylates on a histidine residue and then transfers this phosphate to an aspartate on a second protein. Both of these activities are higher when ...

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…The C-terminal regions of plant PHY and of those prokaryotic PHY that have been sequenced are not homologous (Lamparter, 2004;Mathews, 2006); in green plants, this region comprises two PAS and single His kinase and ATPase domains ( Figure 3B), the latter two domains form the core of the His kinase-related domain (HKRD; Rockwell et al, 2006). The PAS repeats and HKRD contain sites necessary for dimerization and nuclear localization (Quail, 1997;Chen et al, 2003) and for modulating phytochrome signaling (Krall and Reed, 2000;Matsushita et al, 2003;Oka et al, 2004;Mü ller et al, 2009). Sequence conservation among the photosensory cores of plant and Synechocystis 6803 Cph1 phytochromes facilitates their alignment (Essen et al, 2008), and the alignment serves as a preliminary basis for relating positions in plant phytochromes to Cph1 structural elements ( Figure 3C).…”

Genetic suppression of plant development and chloroplast biogenesis via the Snowy Cotyledon 3 and Phytochrome B pathways

“…S2b). This amino acid is conserved in all phytochromes and is located at the beginning of the PAS repeat domain, which is required for nuclear localisation of the PhyB protein (Chen et al 2005;Krall and Reed 2000). Since we observed a segregation of green and pale seedlings in the M2 generation, it is to be expected that the phyB mutation in this line is dominant.…”

“…This was only observed in the sco3-1phyB-9 mutant and not in the sco3-1phyB-XD double mutant. Thus the inference is that this phenotype is influenced by the mutant allele, which, in phyB-9, is close to the binding region of the phytochromobilin chromophore; in phyB-XD, the mutation is within the PAS repeat domain close to the first PAS motif (Krall and Reed 2000). The latter was found to be required for nuclear relocalisation of the PhyB protein whereas phytochromobilin itself is necessary for the perception of light.…”

Function of the N-terminal region of the phosphate-sensing histidine kinase, SphS, in Synechocystis sp. PCC 6803

“…Overaccumulation of histidine kinases and response regulators in cells can cause excess signal transduction between cognate partners or between non-cognate partners. (Ehira & Ohmori, 2006;Krall & Reed, 2000). This phenomenon has been utilized to screen for novel histidine kinase activities.…”