Signal Transduction by Phytochrome: Phytochromes Have a Module Related to the Transmitter Modules of Bacterial Sensor Proteins

Abstract

A C-terminal section of phytochromes turned out to share sequence homologies with the full length of the transmitter modules (about 250 amino acids) of bacterial sensor proteins. Coinciding hydrophobic clusters within the homologous domains imply that the overall folding of the two different types of peptides is similar. Hence, phytochromes appear to possess the structural prerequisites to transmit signals in a way bacterial sensor proteins do. The bacterial sensor proteins are known to be environmental stimuli-regulated kinases belonging to two-component systems. After sensing a stimulus by the N-terminal part of the sensor protein, conformational alterations confer the signal to its (mostly) C-terminal transmitter module which in turn is transitionally autophosphorylated at a conserved histidine. From the histidine the phosphate is transferred to the receiver module of a system-specific regulator protein which eventually acts on transcription or enzyme activity. The histidine is not conserved in phytochromes. Instead, a conserved tyrosine is found spatially very close to the histidine position. This tyrosine might play the role of histidine, and kinase function might be associated with this part of phytochrome. In spite of this divergence, the structural similarities point to a common evolutionary origin of the phytochrome and bacterial modules.

Cryptogam phytochromes

“…Phytochroine has been proposed to function as a protein kinase on the basis of sequence homology to a kinase © 1997 Blackwell Science Ltd. PlatU, Cell atul EtivirotwtetU, 20, 685-690 catalytic domain (Schneider-Poetsch 1992). However, kinase activity of these gene products was only confirmed for PHYCER (Thummler et al 1992;Algarra, Linder & Thlimmler 1993).…”

The pair of bacteriophytochromes from Agrobacterium tumefaciens are histidine kinases with opposing photobiological properties

“…The C-terminal transduction domain of Cphs is strongly related to known two-component sensor kinases, and as expected they behave as histidine kinases in vitro (4). Likewise, phys have a C-terminal domain related to sensor kinases (5,6). However, this domain is missing critical residues essential for catalysis including the positionally conserved histidine essential for phosphotransfer, implying that phys descended from a Cph progenitor but have since acquired a new type of kinase activity.…”

Illuminating Phytochrome Functions (There Is Light at the End of the Tunnel)

“…Recent analyses of phytochrome amino acid sequences from evolutionarily primitive plants have provided some promising clues. SchneiderPoetsch and co-workers noted that the C-terminal domain of phytochrome from the lycopod Selaginella and from higher plants such as Avena, Zea, and Arabidopsis exhibit weak but potentially significant homology with a number of bacterial proteins that act as signal transmitters (Schneider-Poetsch, 1992). These bacterial proteins belong to a group of twocomponent sensors that receive environmental stimuli through an N-terminal module and transmit the information to a C-terminal receiver module via a series of conformational changes.…”

“…In some cases, the C-terminal module then activates other regulatory molecules by protein phosphorylation. By analogy, it has been proposed that phytochrome functions by passing the light signal from the chromophore-bearing Nterminal domain to the C-terminal region, which then activates a second, as yet unidentified, component within the phytochrome signal transduction chain (Schneider-Poetsch, 1992 The recent cloning of a novel phytochrome gene from the moss Ceratodon purpureus has yielded some striking possibilities regarding the nature of the second component (Thiimmler et al, 1992). This gene encodes a 145-kD protein consisting of a natural fusion between phytochrome and a 300-amino acid domain with substantial homology to the catalytic domain of known protein kinases.…”