Phytochrome

Phytochromes are a class of widespread photoreceptor proteins found in plants, algae, bacteria, cyanobacteria and fungi. They measure the ratio of bright red to far-red light and control a wide variety of responses to light stimuli, such as the greening of parts of plants, the shadows flee or seed germination in plants. In addition to the cryptochromes and phototropins they are the most important class of photoreceptors.

Importance

In higher plants, phytochromes control a variety of processes, including germination, seedling photomorphogenesis, flowering, photoperiodism, and avoid green shade as he is to be found under leaf ceilings. In lower plants such as mosses phytochromes are responsible for phototropism and the Polarotropismus.

Cyanobacterial phytochrome are responsible for the chromatic adaptation of the photosynthetic apparatus: the composition of the photosynthetic pigments accessory is adapted to the spectral properties of the light. In bacteria, operating the anoxygenic photosynthesis, the composition of the photosynthetic apparatus is determined.

In some fungi, the phytochrome signal on entry into the vegetative or generative stage of development decisions.

Response to light: photoconversion

Phytochrome can exist in two different conformations: the Pr form (r for red, English red) has an absorption maximum at 660 nm, the Pfr form ( fr for dark, English far red) at 730 nm These spectral properties are both the chromophore, but also by the surrounding protein determined.

It was recognized early that can alter the absorption properties of the molecule and also the plant responses by exposure to certain light: include grown in the dark fabric, only the physiologically inactive Pr form of phytochrome, which after exposure to red light in the active Pfr form passes. The plant then displays typical reactions such as photomorphogenesis. By irradiation with far-red light, this effect can be reversed - Pfr goes back into the inactive Pr form. Pfr returns through its thermodynamic instability independent of light back to the Pr form. This process is called Dunkelreversion or dark conversion.

The biological responses are determined by the ratio Pr / Pfr and the extent of Dunkelreversion.

This process is initiated by a conformational change of the chromophore upon irradiation with appropriate lighting that draws a structural change of the whole protein by itself. The kinase activity of the protein is changed. In plants, the widely separated domains P3/GAF and PAS -A and PAS - B are exposed. They carry signals for the transport into the nucleus and possibly interaction surfaces for partner proteins.

Protein structure

Phytochromes are highly conserved in many parts, all is an N-terminal sensor region and a C -terminal regulatory and dimerization in common.

The sensor region is comprised of three domains P2/PAS, P3/GAF and P4/PHY, wherein the chromophore in bacteria and plants in the P2/PAS the P3/GAF-Domäne with the A- ring via a thioester linkage to covalently a cysteine ​​of the protein is bound and is sunk in a deep pocket. In plants, there is furthermore an additional N -terminal P1 domain, which has different functions in different species. P2/PAS P3/GAF and are essential for the light perception and signal transduction have bilin- lyase activity, which is required for the incorporation of the chromophore. P4/PHY is responsible for the spectral characteristics of the kinase activity, and to fine-tune the activity of the phytochrome and causes in the transport in the plant nucleus.

The regulatory region has in its original form a domain with histidine kinase activity, with the help of proteins can be phosphorylated. In plants, this function is lost, and how the kinase activity is brought about by a serine-threonine kinase that has been found in the N- terminus of the protein. Plants have additional PAS A and PAS -B domain, which are responsible for the dimerization and transport into the nucleus.

In eukaryotes phytochromes always occur as dimers, in prokaryotes only as a monomer.

Structure of the chromophore in plants

Depending on the group of organisms can be found several chromophores derived from heme. Bacteria and fungi use biliverdin IX α, higher plants phytochromobilin, cyanobacteria and algae phycocyanobilin.

The chromophore is a linear tetrapyrrole, which is fixed to the rings A, B and C by its environment. As a result of photoconversion of the D- ring can rotate after receiving a bright red light quantum. The molecule is as a cis -trans isomer form on the double bond between C15 and C16. PR - form of the molecule is C15 -Z, anti- configured, the Pfr form has C15 -E, anti- configuration. The conformational change of the chromophore causes a conformational change of the whole protein by itself, thus the transitions from Pr to the Pfr form. This process is reversed by irradiation with far-red light.

Gene family of plant phytochromes

In Arabidopsis thaliana, five genes were previously found for phytochrome, three in rice, four in the jaw and three in Ginkgo biloba. Phylogenetic analyzes have shown that the phytochromes have split before the divergence of seed plants in the two main groups phy A and phy B, can be classified in the all phytochromes. In Ceratodon purpureus, a moss, 4 phytochromes are known so far, one of them has a unique gene sequence for phytochrome so far, but has not yet been clarified whether it is in this version is a functional protein or a pseudogene.

Phy A is found almost exclusively in large amounts in etiolated plants before because its Pfr form is unstable and its own gene transcription inhibits. Phy B occurs in solid under normal light conditions plants, and in small amounts in etiolated plants. The Pfr form of phy B is stable in contrast to Phy A and is constitutively expressed.

Many of phytochrome have similar characteristics and will share certain responsibilities, as well as many of them have specialized roles in their biological function.

Signal transduction

The intracellular signal transduction is largely unknown. But it is to be assumed that the domain in prokaryotes Histokinase plays an important role in the transfer of phosphate residues. In eukaryotes is no more Histokinase activity, however, a serine-threonine kinase activity is present.

In eukaryotes, a translocation of the protein is detected in the nucleus where it can enter into interactions with transcription factors and directly affect gene expression there. However, it can also be generated cytosolic answers.