Life is tough out there. If chemistry (via oxidizing radicals) doesn’t get you, sunlight (via UV irradiation) will. No wonder that cells, both prokaryotic and eukaryotic, have an intense preoccupation with keeping their DNA intact. In most organisms, a considerable portion of the genome is devoted to making sure that this actually takes place. There are two good ways to do this. One is to repair damaged DNA, the other to re-acquire undamaged DNA from undamaged cells by homologous recombination. The two are biochemically the same process, varying only in the context of how they are used. But for prokaryotes to acquire undamaged DNA from another cell, donor and recipient cells must conjugate, and that doesn't grow on trees. To make sure that it takes place with some frequency, some archaea have evolved a neat intracellular signaling system. The punch line is that they respond to UV irradiation by activating their cell-cell conjugation system.
by Moselio Schaechter
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