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Gilbert_RNAWorld_Nature_1986

Gilbert_RNAWorld_Nature_1986 - 618 Origin of life NATURE...

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Unformatted text preview: 618 Origin of life NATURE VOL. 319 20 FEBRUARY 1986. NEWS AND VIEWS The RNA world from Walter Gilbert UNTlL recently, when one thought of the varied molecular processes at the origin of life, one imagined that the first self- replicating systems consisted of both RNA and protein. RNA served to hold information, whereas protein molecules provided all the enzymic activities needed to make copies of RNA and to reproduce themselves. The cycle that developed a ' self-replicating system out of the primitive soup of amino acids and nucleotides had two radically different components‘. Now it seems possible that the informa- tional and catalytic properties of these two components may be combined in a single molecular species. Last week in these col- umns Frank Westbeimer2 described the discovery of enzymic activities in the RNA molecules of Escherichia coli, in which ribonuclease—P cuts phosphodiester bonds during the maturation of the transfer RNA molecule“, and of Tetrahymeria, whose ribosomal RNA contains a self- splicing exon”. If there are two enzymic activities associated with RNA, there may be more. And if there are activities among these RNA enzymes, or ribozymes, that can catalyse the synthesis of a new RNA molecule from precursors and an RNA template, then there is no need for protein enzymes at the beginning of evolution. One can contemplate an RNA world, con- taining only RNA molecules that serve to catalyse the synthesis of themselves“. The self—splicing intron is an RNA ele- ment that can splice itself out of an RNA molecule. This reaction should be reversi- ble, and the intron could splice itself back into an appropriate nucleotide sequence. Thus, in the RNA world, such introns could both remove and insert themselves into the background of replicating RNA molecules. The significance of this is not the simple insertion and removal of in- trons, but the fact that two introns, sepa— rated from each other by another RNA element, an exon, can combine with each other so as to remove as a unit both them~ selves and the intervening exon from one RNA molecule and to insert into another. Thus, self-inserting introns can create transposons to move exons around. This property provides RNA with a major evo» lutionary facility that it otherwise lacks ——— recombination, the ability to produce new combinations of genes. Of course the self replicating molecules would in any case have evolved slowly by miscopying, that is, by mutation. But transposons provide the equivalent of sex —« the infectious transmission of genetic elements from one organism to another. Recombination and sex are powerful devices to permit a more useful exon to pass from one replicating structure to an unrelated one. This picture of the RNA world is one of replicating molecules that reassort exons by transposable elements created by in- trons. This process builds and remakes RNA molecules by chunks and also per— mits the useful distinction between in— formation and function. Information stor— age needs to be one-dimensional, for ease of copying, but molecules with enzymic functions tend to be tight three- dimensional structures, whose forms are unrelated to the demands of any copying mechanism. (This dichotomy is most ob- vious today between the linear order along DNA and the structure of proteins.) In the RNA world, the structure that would be replicated has the full comple- ment of introns. Some of the daughters, by splicing out all their introns, would con- vert to functional molecules, the ribozy- mes. A remnant of this process may be the structure of transfer RNA, where a com— pact secondary structure is b‘roken up by the insertion of an intron. The first stage of evolution proceeds, then, by RNA molecules performing the catalytic activities necessary to assemble themselves from a nucleotide soup. The RNA molecules evolve in self—replicating patterns, using recombination and muta‘ tion to explore new functions and to adapt to new niches. By using RNA cofactors, such as nicotinamide adenine dinucleotide and flavin mononucleotide they then de« velop an entire range of enzymic activities”. At the next stage, RNA mole- cules began to synthesize proteins, first by developing RNA adapter molecules that can bind activated amino acids and then by arranging them according to an RNA" template using other RNA molecules Such ’y as the RNA core of the ribosome. This process would make the first protein which would simply be better enzymes than their RNA counterparts. I suggeg; that protein molecules do not carry Out 8, enzymic reactions of a different nan”e .1 from RNA molecules but are able to PEr- ' form the same reactions more effective!y and rapidly, and hence will eventual}V dominate. These protein enzymes are Enl- coded by RNA exons, thus they, in turn" are built up of mini-elements of structure, Finally, DNA appeared on the scene. the ultimate holder of information cop: ied from the genetic RNA molecules by reverse transcription. After double. stranded DNA evolved there exists a stable linear information store, ermr. correcting because of its double—stranded structure but still capable of mutation and recombination. RNA is then relegated to the intermediate role that it has today \ no longer the centre of the stage, dig. placed by DNA and the more effective protein enzymes. But a few RNA enzymic activities still exist, the two described recently3 ‘7, and possibly others in the role of ribosomal RNA or in the splicing of eukaryotic messenger RNA. The relic of this process is the intron/exon structure of genes, left imprinted on DNA from the RNA molecules that earlier encoded pro teins, a residue of the basic mechanism of RNA recombination. 3 I. Eigen, M., Gardiner, W., Schuster, P. & Winkler~ Oswatitsch, R. Sci. Am. 244, 88 (1981). 2. Westheimer, EH. Nature 319, 534 (1986). Guerrier-Takada, C., Gardiner, K., Marsh, T., Pace, N.& Altman, S. Cell 35, 849 (1983). GuerriervTakada, C. , & Altman, S. Science 223, 285 (1984). Kruger, K. elal, Ccll3l, 147 (1982). Cech, TR. Int. Rev. Cytol. 93, 3 (1985). Zaug, AJ, & Cech, TR. Science 231, 470(1986). Sharp, P.A. Cell 42, 397 (1985). White, H.B. Ill 1. molec. Evol. 7, 101 (1976). l" EOWFR‘P‘.“ Walter Gilbert is at the Biological Laboratories, Harvard University, 16 Divinity Avenue, Cam‘ - bridge, Massachusetts 02138, USA. Infrared detectors Superlattices point ahead from Gordon C. Osbourn TECHNICAL developments in the past few years have made clear what the next gen- eration of infrared detectors will be like. The need is for semiconductor materials appropriate for long-wavelength (2 12 um) imaging, at the focal planes of new telescopes and for various defence ap plications. Most effort has focused on the II~VI alloy system (Hg,Cd)Te, but work has also begun on a recently proposed III—V system consisting of many alternat- ing layers of thin mismatched 1n(As,Sb) crystal layers, called strained-layer super- lattices (SLSs). A crucial first step in their development has now been achieved with the successful growth of crystals of SLSs in the In(As,Sb) alloy system”. The materials from which long-wave- length detectors might be made are limit~ ed by the stringency of the requirements. These imaging devices will consist of two— dimensional arrays of many individual photovoltaic detector elements on a single wafer of a material which must have an energy bandgap less than or equal to 0.1 eV at about 77 K, the operating temperature of the infrared detector. This bandgap is smaller than that of all the III—V allo)I “W w, NAT mm lle‘ will haV wh} But ma' be ' mo tor thif (H2 mu Th: wit cor on ter sta dit en‘ str lar att on car ...
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