Replicase ribozym

In a similar vain, Szostak et al. [17] propose that a protocell composed of a growing membrane, a general replicase ribozyme (able to replicate also another copy of itself), and another ribozyme involved at some stage in membrane formation would be truly alive. Once again, it is clear that this system is an ultimate heterotroph [19], completely devoid of a metabolic sub-... 

No. Despite much work to find a self-replicating RNA, the replication always requires the presence of protein. Recent work by David P. Bartel at MIT is showing some promise toward finding an RNA replicase ribozyme (Science 292[5520] 1319). The fact that an RNA replicase may be produced in the laboratory does not, of course, prove that the ribozyme existed in nature. 

Szostak et al. worked on the basis of a simple cellular system which can replicate itself autonomously and which is subject to Darwinian evolution. This simple protocell consists of an RNA replicase, which replicates in a self-replicating vesicle. If this system can take up small molecules from its environment (a type of feeding ), i.e., precursors which are required for membrane construction and RNA synthesis, the protocells will grow and divide. The result should be the formation of improved replicases. Improved chances of survival are only likely if a sequence, coded by RNA, leads to better growth or replication of membrane components, e.g., by means of a ribozyme which catalyses the synthesis of amphiphilic lipids (Figs. 10.8 and 10.9). We can expect further important advances in the near future from this combination ( RNA + lipid world ). 

Fig. 10.9 Possible reaction pathway for the formation of a cell. The important precursors are an RNA replicase and a self-replicating vesicle. The combination of these two in a protocell leads to a rapid, evolutionary optimisation of the replicase. The cellular structure is completed if an RNA-coded molecular species, for example, a lipid-synthesised ribozyme, is added to the system (Szostak et al., 2001)... 

This is illustrated in Figure 11.3. It consists of a vesicle containing two ribozymes, one (Rib-2) capable of catalyzing the synthesis of the membrane component the other (Ribl) being an RNA replicase that is capable of repUcating itself, and reproducing the Rib-2 as well. In this way, there is a concerted shell-and-core replication, and there is therefore a basic metabolism, self-reproduction, and - since the replication mechanism is based on RNA replication - also evolvability. 

Figure 11.4 The hypothetical pathway for the transformation of a simple RNA cell into a minimal DNA/protein cell. At the first step, the cell contains two ribozymes, Rib-1 and Rib-2 Rib-1 is a RNA replicase capable of reproducing itself and making copies of Rib-2, a ribozyme capable of synthesizing the cell membrane by converting precursor A to surfactant S. During replication, Rib-1 is capable of evolving into novel ribozymes that make the peptide bond (Rib-3) or DNA (Rib-4). In this illustration, these two mutations are assumed to take place in different compartments, which then fuse with each other to yield a protein/DNA minimal cell. Of course, a scheme can be proposed in which both Rib-3 and Rib-4 are generated in the same compartment. (Modified fromLuisi et al., 2002.)...

RNA catalysis is not only concerned with RNA cleavage non-natural ribozymes that show ligase activity (Bartel and Szostak, 1993) were obtained and many (so far not yet successful) efforts have been undertaken to prepare a ribozyme with RNA replicase activity. RNA catalysis does not only operate on RNA, nor do nucleic acid catalysts require the ribose backbone. Ribozymes were trained by evolutionary techniques to process DNA rather than their natural RNA substrate (Beaudry and Joyce, 1992), and catalytically active DNA molecules were evolved as well (Breaker and Joyce, 1994 Cuenoud and Szostak, 1995). Systematic studies revealed many other examples of RNA catalysis on non-nucleic acid substrates (see... 

The replication paradigm requires that protein enzymes were not present at the beginning, and RNA replication was therefore performed by ribozymes. Some RNAs can in fact behave as polymerases and replicases, but they are far less efficient than the corresponding protein enzymes, and the accuracy of their replications was necessarily very low. The experimental measures, obtained from interacting coupled nucleotides, have shown that without protein enzymes the replication error e cannot be less than 0.01, which means, from formula 5.1, that primitive RNAs could not have, as an order of magnitude, more than 100 nucleotides (Maynard Smith and Szathmary, 1995). 

So far, we have constructed an unsatisfying picture of the earliest days of an RNA world although some prebiotic mechanisms may exist for the untemplated formation of oligonucleotides, these molecules would have been short, would have contained a variety of monomers besides ribotides, and could not have been faithfully copied by the template-directed polymerization of monomers. Given this model, it is difficult to imagine the accumulation of RNA sequences necessary for the Darwinian selection of a multitude of active ribozymes. Nevertheless, these precursors may have been adequate for the first critical step in the formation of life the formation of an RNA replicase. 

It has already been pointed out that a great deal of intracellular biochemistry is based on cofactors, with these cofactors, in turn, often being derived from nucleotides. However, while this indirectly implies the proficiency of ancient RNA catalysts, it does not prove that such catalysts could have existed. Although there are, for example, protein dehydrogenases and esterases, there are no modem ribozymes with similar activities. Just as engineering a ribozyme self-replicase will be an experimental demonstration that life could have arose via RNA, so the production of artificial ribozymes will be a demonstration that a metabolically complex RNA world may once have existed. 

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