A research team jointly involving the IRD, the CEA and the CNRS has very recently found phytochromes in a strain of nitrogen-fixing bacterium, Bradyrhizobium (1), symbiont on certain tropical leguminous plants (the Aeschynomene). Techniques of molecular biology, biophysics and biochemistry revealed that the newly-discovered phytochrome has an essential role as regulator of the bacterium’s photosystem synthesis. An identical function was shown in the photosynthetic bacterium Rhodopseudomonas palustris, phylogenetically very close to Bradyrhizobium (2).
The researchers experimented by subjecting Bradyrhizobium cells to different wavelengths of light, from the red to the infrared. It appeared that the bacterial photosynthetic apparatus was synthesized in its complete form only when the phytochrome was in its active (far-red-light absorbing) configuration (3). In addition, they used genetic engineering techniques to make bacterial strains in which the gene coding for the phytochrome was suppressed. These strains showed practically no photosynthetic activity whatever the culture conditions. These experiments therefore demonstrated that the photosystem of Bradyrhizobium is totally under the control of the bacteriophytochrome. This is the first time that any definite role has been determined for phytochromes in bacteria. Another positive result was the determination of the main action mechanisms of the phytochrome in these bacteria. The gene adjacent to that of the phytochrome encodes a protein (called transcriptional factor “ PpsR ”) already known to repress the expression of some photosynthetic genes (4). The team demonstrated that when in its active form under infrared light, the phytochrome interacts with this protein and stops its repressive action. The genes which encode the bacteria’s photosynthetic apparatus can then express themselves. In this way, the light signal transduction the phytochrome ensures in the bacterial cells would occur by direct interaction with PpsR, meaning a direct protein-protein interaction mechanism and not the induction of a biochemical reaction (phosphorelay) cascade, which has been the theory up to now. The researchers used these observations to devise a model for gene expression control by light. A patent has been filed for this model which could be useful as a new research tool in molecular biology (5).
The crucial question here is why these bacteria of the Bradyrhizobium genus should be equipped with phytochromes whereas other photosynthetic bacteria (Rhodobacter, Rubrivivax or Rhodospirillum) analysed by the IRD, the CEA and the CNRS have none. The hypothesis the researchers advance is that the phytochrome’s photosynthesis control system could represent a function-based ecological adaptation that allows interaction between the bacterium and the leguminous plant on which the bacterium is developing. The Bradyrhizobium bacterium can implant itself along stems under a layer of chlorophyllous cells which let through only infrared wavelengths Thus, the phytochrome enables the bacterium to install its photosynthetic apparatus. That will then supply part of its energy requirement for maintaining its symbiosis with the leguminous plant and fixing the nitrogen essential for the plant’s growth.
The study of phytochromes in photosynthetic bacteria could in the long term bring a better understanding of the operational mechanisms of these light sensors in plants. Rhodopseudomonas palustris, the other bacterium studied by the IRD, the CEA and the CNRS, is a particularly suitable model for analysing phytochrome function in general. The entire genome of this bacterium has recently been sequenced and shown to contain six different copies of phytochromes, which is exceptional.
(1) In other words, they use light as an energy source both for their own growth and to enable the symbiosis with the leguminous plants to operate and fix the nitrogen these plants need for their development.
(2) R. palustris is known to microbiologists as one of the most versatile bacteria, capable of adapting its metabolism to highly varied environmental conditions.
(3) Most bacterial phytochromes so far identified are active under infrared light, unlike plant phytochromes which sensitive to red light.
(4) This protein, termed PpsR, has been isolated from several other micro-organisms. It recognizes a particular region of DNA upstream of the gene it controls. The protein fixes on this region, thus preventing the passage of RNA polymerase and blocking transcription.
(5) See the press release issued jointly by CEA/CNRS/IRD.
Journal
Nature