Scientists from the Dept. of Plant Physiology in Giessen and the Dept. of Structural Biology in Marburg have made an important step towards understanding how "phytochromes" work. This is described in a publication in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) entitled "The structure of a complete phytochrome sensory module in the Pr ground state" due to appear shortly.

The phytochrome molecule is a remarkable light-activated switch which in plants triggers germination, regulates stem extension, development of the photosynthetic apparatus, responses to shading from competitors and even the induction of flowering - indeed the most radical environmental effects on development known in all biology. Unfortunately, how exactly phytochromes work is still only partly understood, even though numerous laboratories worldwide devote their energies towards the question. Up to 1996 it had been thought that phytochromes only existed in plants, but at that time Jon Hughes - then a postdoc at the Free University of Berlin and now Professor of Plant Physiology at Giessen - and colleagues discovered the first prokaryotic phytochrome in the photosynthetic cyanobacterium Synechocystis 6803. That work, published in the renowned journal "Nature", had far-reaching implications in this research field, in particular because it proved much easier to work with than phytochrome from plants. Three years ago, Prof. Hughes's group teamed up with the Prof. Lars-Oliver Essen's Structural Biology laboratory at the University of Marburg to try to crystallise Cph1. The purpose behing this was to determine the 3D structure of the Cph1 molecule via X-ray crystallography. Indeed, the scientists were quickly successful, so that the exact position of almost every atom in the molecule is now known. Through this it becomes possible to understand how the absorption of a photon leads to a reorganisation of the molecule which in turn communicates with the control systems of the cell, finally to modify the physiology of the plant as a whole.

The structure gives tantalising insights into the optical switch. Essentially the protein molecule consists of two unequal lobes connected by a long helical rod, rather like a classical telephone handset with a larger earpiece and a smaller mouthpiece. The larger lobe contains the chlorophyll-like pigment co-factor responsible for light absorption. The smaller lobe carries a remarkable tongue-like structure which reaches back to and makes intimate contact with the larger lobe containing the pigment. It seems likely that the tongue somehow is responsible for detecting when the pigment absorbs a photon and transmitting this information to the cell. How exactly this happens remains to be discovered, however. Intriguingly, similar handset-like structures are know in two classes of enzymes involved in cyclic nucleotide signalling in bacteria and animals - one of these enzymes is the target of Viagra! It might be that phytochromes are connected to related signalling systems. In any case, the new structure gives new insights into how phytochromes are constructed and how they might work, information which help us to understand how phytochromes regulate plant development.
Figure 1: Photograph of the crystals:
With the help of precision robots, painstaking screening procedures carried out in darkness using infra-red video equipment and thousands of trials, the scientists were able to find conditions under which Cph1 crystals form. As phytochromes absorb red light, the crystals appear turquoise. In such crystals the molecules are arranged symmetrically, so that they diffract X-rays. The diffraction pattern obtained gives detailed information regarding the structure of the molecule itself. Using the X-ray beamlines at the synchrotrons in Grenoble and Hamburg and following complex mathematical procedures it proved possible to solve the structure of the molecule.
Source: Justus-Liebig-Universität Gießen
Figure 2: Model of the molecule:
The Cph1 "telephone handset" consists of a larger "earpiece" lobe (above, in green, blue and gold), in which the small pigment co-factor (turquoise) is visible, and a smaller "mouthpiece" lobe (below, in red). The lobes are connected by a long helical rod (gold-red on the right side), while a remarkable "tongue" (red, left middle) protrudes from the lower lobe to make contact with upper lobe. The scientists suspect that the tongue acts as a delicate sensor for light absorption by the pigment co-factor.
Source: Justus-Liebig-Universität Gießen

The work was financed by grants from the Deutsche Forschungsgemeinschaft.

Contact:
Prof. Jon Hughes, BSc, PhD
Pflanzenphysiologie
Senckenbergstr. 3
35390 Gießen
Germany
Telefon: 0641 99-35430
Fax: 0641 99-35429
E-mail: jon.hughes@uni-giessen.de