Photosynthetic pigment and evolution
There are various kinds of photo-environments on the earth, such as the rocky top of the Alps, which receives strong sun rays, places under water at a depth of 150 meters, which receive only 1 % of the sun light that the surface of the earth receives, and so on. The organisms with a photosynthetic system exist in almost all these various environments. In order to live in these different environments, plants must have pigment that suits the color and strength in these environments. In fact, there is a great variety of pigment in the organisms which have a photosynthetic system. To compare those pigments, and to know the evolution of pigment are very important for understanding the evolution of plants. How did plants obtain new pigment? In this article, I use chlorophyll a and chlorophyll b as examples to explain the evolution of photosynthetic pigment.
Study the diversity of pigment through the examination of molecular system
Among Prokaryote which have a photosynthetic system just as plants growing in soil do, Cyanobacteria (blue-green algae) which have chlorophyll a and prokaryotic chlorophyta, which have both chlorophyll a and b, are known. Chlorophyll b is found in plants growing in soil and in eukaryotic chlorophyta, which is the origin of plants, but it is not found in Eukaryote such as red and brown algae. Where did chlorophyll b come from and how did it diversify within those organisms? Since it is known that chlorophyll b is made from chlorophyll a through two enzyme reactions, I applied lineage analysis using the CAO gene, which is an enzyme gene related to the above reactions. The results showed that the CAO genes of prokaryotic and eukaryotic chlorophyta, and green plants were not obtained individually, but rather originated from a common ancestor. In other words, the common ancestor of prokaryotic chlorophyta and blue-green algae had both chlorophyll a and b, and blue-green, red, and brown algae have somehow lost chlorophyll b in the evolutional process. The process described above is different from the process that was drawn from the result obtained by the ordinary method. This points out that a new biocoenosis was established not by obtaining the special photosynthetic pigment, but by losing it.
Reproduce the photosynthetic evolution in the laboratory
The combination of photosynthetic pigment and protein form a complex. If the pigment’s arrangement or its kind changes, the complex can’t use photo-energy. Therefore, it was believed that the arrangement and kind of the pigment in the protein were strictly determined. But when I thought about it more, I came up with two questions, that is, in the process of evolution, how photosynthetic plants predicted new pigment and how they were able to prepare the best suitable protein for pigment in advance. Besides, in many cases, isolated pigment from protein produces active oxygen; therefore, it is a very dangerous molecule to organisms. These points have made understanding of the process of obtaining pigment, difficult. By the analysis of molecular genealogy, we were able to find the evolutional course of the pigment system. But this fact is not evidence enough to understand, in a practical way, how new pigment is taken into protein and how it functions there. Therefore, I put the CAO gene into the algae which didn’t have chlorophyll b, and examined what would happen in the cells when chlorophyll b entered them for the first time. As I mentioned earlier, isolated chlorophyll b is toxic to cells. So I thought algae with the CAO gene would die immediately. But contrary to my expectations, the algae multiplied just like the wild type did. Therefore, I examined in detail how chlorophyll b existed in the cell. The result was this: chlorophyll b was taken into the protein which was originally thought not able to bind with chlorophyll b, and the protein obtained photo-energy. The protein, then, delivered photo-energy to the nearby pigment, in the usual way. From these findings, evolution may have occurred as follows. At first, the prokaryon type photosynthetic organism, which had chlorophyll a, obtained the CAO gene, and then synthesized chlorophyll b. Synthesized chlorophyll b was taken into the already existing protein, and then the protein became able to use the light wavelength, which the protein could not originally use, for photosynthesis. Later on, the binding type protein, which exists currently and binds with chlorophyll b more practically, was born. As a result of this, chlorophyll b was taken into this protein, which became the current pigment style. This is my scenario of the birth of the current pigment style. This unexpected scenario was obtained from the experiment of the CAO gene transduction. But this experiment can be applied not only to experiments on the process of obtaining chlorophyll b but also to other events, which appeared on the evolutional course of photosynthesis. And I believe that this experiment will be a very effective procedure in obtaining a clear picture of photosynthesis in the future.
[The reappearance experiment of evolution?]
The position of the coloring matter in reaction center protein was decided strictly. Antenna coloring matter is carrying out the role which passes energy to reaction center coloring matter (two coloring matter combines specially) for the received optical energy. When a CAO gene is introduced into SHIANO bacteria without Chlorophyll b and Chlorophyll b was made to make, it turns out that it is taken in by reaction center protein and functions exactly.
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Quarterly journal Biohistory:vol.30
