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From Nature
Using just two bases, Reader and Joyce mimicked the R3 ligase ribozyme, a stretch of RNA that latches onto another RNA molecule. Part of the R3 ribozyme has a base sequence that matches that on its RNA target molecule. The researchers constructed this binding sequence and target from A and U bases alone. Then, for technical reasons, they replaced the A's with a non-natural base called diaminopurine (D). The resultant ribozyme can be copied without the need for G or C bases. Copying is a necessary part of the process of finding a two-letter mimic. The researchers then eliminated all of the G's that they could from the R3 molecule while still retaining some of its catalytic behaviour (it can manage without C's). All but three could go; if the researchers took any of those out, the molecule was no longer catalytic. To get further, the researchers abandoned rational design and turned to in vitro evolution. They replaced the remaining G's at random with U or D, while shuffling a few of the other U's and D's in the molecule. None of the products made this way is a particularly stunning catalyst. But they work. The best, containing just U and D, links to the RNA target 36,000 times faster than in the absence of any catalyst at all. In other words, a two-letter ribozyme is a lot better than nothing. D isn't too difficult to create from the kind of ingredients that were probably available on the early Earth, say Reader and Joyce. They also point out the advantage of an RNA-like molecule that contains no C: cytosine decomposes quite quickly if it gets warm. |