Chloroplasts, the plant cell’s green solar power generators, were once living beings in their own right. This changed about one billion years ago, when they were swallowed up, but not digested, by larger cells. Since then, they have lost much of their autonomy. As time went on, most of their genetic information found its way into the cell nucleus; today, chloroplasts would no longer be able to live outside their host cell. Scientists on Dr. Ralph Bock’s team at the Max Planck Institute of Molecular Plant Physiology in Germany have discovered that chloroplast genes take a direct route to the cell nucleus, where they can be correctly read in spite of their architectural differences. Their results were published online on April 12, 2012 in Current Biology. Cyanobacteria are among the oldest life forms, and appear to be the forerunners of green chloroplasts in plant cells. They do not possess a true cell nucleus, but their genetic substance is made up of the same four building blocks as that of humans, plants, and animals. Therefore, the genes encoded in the chloroplast DNA can also be read in the cell nucleus; indeed, many genes that were still found in the cell organelles during early evolution are now located exclusively in the genome of the nucleus. How they made their way there has previously been unclear. Two mechanisms appeared likely: either direct transport in the form of DNA fragments from the chloroplasts to the nucleus or transport in the form of mRNA, which is then transcribed back into DNA. The direct transfer of DNA appears to predominate in the chloroplasts, but this pathway raises two problems. The first problem lies in the promoters, the DNA sequences which ensure that genes are recognized as such.
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