Convergence of different macromolecular structures with same function

[Jarven]

My professor had given us a lecture on the RNA World Hypothesis. He provided evidence for this hypothesis by citing the in-vitro selection experiments carried out on RNA.

The outcome of one (or more) of these experiments resulted in the "creation" of an RNA polymerase ribozyme which totally blew my mind. It was capable of ligating 10-15 nucleotides on RNA oligonucleotides, which is actually really terrible processivity (but its a goddamn RIBOZYME).

A paper had recently solved the structure of this ribozyme and my question to the professor was whether there were any similarities between the ribozyme structure and that of an RNA polymerase. He quickly dismissed my question, stating that the ribozyme is unlike anything seen before :(

This actually got me thinking, would different macromolecules (in this case RNA and proteins) converge to similar structures over a long period of time (through evolutionary processes) if they were catalyzing the same reaction?

I had difficulty finding papers that compared the structures of ribozymes and proteins with similar function.

 

[Ygggdrasil]

Nature has performed this experiment with RNA polymerase proteins. Most organisms use a multisubunit RNA polymerase to transcribe DNA into mRNA. Despite the similarities between DNA synthesis and RNA synthesis, these multisubunit RNA polymerases are evolutionarily and structurally unrelated to DNA polymerases. Some viruses, however, contain single subunit RNA polymerases (one of the best studied examples is T7 RNA polymerase from the bacteriophage T7). Because these single subunit RNA polymerases evolved from DNA polymerases, they are unrelated to the multisubunit RNA polymerases in both sequence and structure. While there are some similarities (they use essentially the same catalytic mechanism), there are also major differences in the way these two families of RNA polymerases work (for example, they interact with their substrates in very different ways). Therefore, despite performing exactly the same reaction (RNA synthesis) and performing the same cellular role (transcription), these two families of enzymes have evolved two very different structures. So, if these differences in structure can exist for two classes of proteins catalyzing the same reaction, I would imaging that such differences would also exist between ribozymes and proteins catalyzing the same reaction.

There are, however, a few examples of protein mimicking RNA structures, for example, the case of the ribosomal release factor proteins very closely resembling the structure of tRNA molecules.