Ribozyme and RNA World Hypothesis | Cell Biology

The below mentioned article provides a study note on Ribozyme and RNA World Hypothesis.

According to this hypothesis primordial cells lacking protein synthesis use RNA both as the repository of genetic information and as enzymes that catalyse metabolism. Enzymatic catalysis which was earlier thought to be the exclusive domain of proteins, but is now established that RNA molecules can cut, splice and assemble themselves without any outside help.

These RNA molecules working as enzymes were called ribozymes. Cech (1982) while working on spli­cing mechanism of RNA molecules during pro­cessing of hnRNA into mRNA or precursor rRNA into rRNA in Tetrahymena, found that removal of proteinaceous enzymes one by one from a cell-free system did not alter the splicing ability of pre-ribosomal RNA.

Later in 1984, Altman using ribonuclease Pi (RNA + Protein) in E. coli demonstrated that catalytic activity of the enzyme resided only in RNA subunit and protein only helped RNA to perform the enzymatic func­tion. Self-cleavage of viroid RNAs or virusoids (satellite RNAs) and catalytic activity of RNase are other examples of ribozymes.

Subsequently, it was shown that ribozymes may also bring about cleavage of DNA molecules and selective­ly amplify RNA molecules.

The discovery of RNA molecules working as enzymes has also changed our idea about origin of cell. It is now believed that four billion years ago, earth was an ‘RNA world’ in which RNA molecules carried out all the processes of life without the help of either protein or DNA.

RNA World:

Early living cells were RNA life-forms. A number of facts suggest that heredi­ty was originally based on RNA rather than on DNA. Due to difficulty in synthesizing nucleotides in ‘primordial soup experiments’ it has been speculated that there was an even ear­lier, although chemically related, precursor to RNA that was a carrier of genetic information.

Arguments in favour are:

1. RNA polymerization and replication can take place in vitro in a purely chemical sys­tem based on nucleotides.

2. The genome of certain viruses is based on RNA.

3. RNA is more complex than stable double stranded DNA because single stranded RNA can fold up in different three dimensional configurations (e.g., tRNA).

4. RNA can have a catalytic effect, so called ribozymes catalyse cutting and splicing of other RNA molecules. RNA molecule even catalyses the polymerization and replication of RNA.

5. The universal and central importance of RNA in protein synthesis.

6. Synthesis of deoxyribose of DNA takes place via ribose of RNA indicating that DNA rep­resents a further and later development of RNA.

7. Nucleosides function as co-enzymes to many enzymes, e.g., ATP. This role of nucleotides can be incorporated as a relict from the RNA world.

RNA molecules were the most important entities in the primitive structural organisation tRNA and rRNA possess a folded three-dimensional structure that may have participated in making informational proteins. Primitive ribosomes were without any protein and capable of making peptides with the help of tRNA that could bind with rRNA without the involvement of enzyme system.

tRNA molecule could link with a given amino acid at the anticodon site through non-covalent linkage indicating enzyme­ like role of primitive tRNA and rRNA.

Linking another RNA strand with tRNA and rRNA for error-free amino acid sequence resulted in the development of template system with the forma­tion of first mRNA. The formation of catalytic protein may have followed the synthesis of nucleoprotein com­plexes. Later developed the enzyme system catalysing the formation of peptide bonds for efficient translation.

As nucleic acids can serve as template in the absence of enzymes and pro­teins, a complementary nucleic acid may have been formed in the presence of a suitable con­densing agent. This led to the formation of DNA which had the self-replicating property and permanent information-storage capacity.

It is possible to study RNA world in the labo­ratory. RNA isolated from virus (Q_β) replicates in a test tube in the presence of an RNA replicase, and four nucleoside triphosphates (ATP, UTP, CTP and GTP). Such RNA culture could at intervals be transferred to fresh medium so that its development could be followed over longer time periods (Fig. 2.7).

Further experiments (Eigen) showed that without the help of RNA primer, in a system containing RNA replicase enzyme, four nucleoside triphos­phates and Mg⁺⁺, RNA strands appeared and replicated.

The RNA molecules formed con­tained about a hundreds of nucleotides and due to internal base pairing they have a hairpin-like shape (Fig. 2.8) termed as quasi-species. Though the replication of RNA depends on the presence of the complex enzyme RNA replicase but attempts have been made by Orgel to make replication of short RNA molecules in enzyme- free system. Thouston et al. have produced an

RNA molecule that catalyses the replication of RNA (ribozyme). The RNA world hypothesis pre­sents a real advance in attempts to understand the origin of cell.

Proto-Cell in RNA World:

RNA replicase is a key component of proto-cell, that can act both as a template for the storage and transmission of genetic information, and as an RNA polymerase that can replicate its own sequence. Simple proto-cell consists of an RNA replicase replicating inside a replicating membrane vesicle (Fig. 2.9).

Both these compo­nents are self-assembling; as RNA molecules can become encapsulated in vesicles as they form, the proto-cell as a whole could self-assemble.

With compartmentation the replicase is capable of vari­ation and natural selection and then Darwinian evolution. An RNA coded activity (e.g., ribozyme) is needed that inputs an advantage in growth of membrane component. A single cell with inter­dependent genome and membrane would be a sustainable, autonomously replicating system.