Role of messenger RNA in binding of peptidyl transfer RNA to 30-S and 50-S ribosomal subunits

The transition from noncoded to coded protein synthesis: did coding mRNAs arise from stability-enhancing binding partners to tRNA?

BACKGROUND

Understanding the origin of protein synthesis has been notoriously difficult. We have taken as a starting premise Wolf and Koonin's view that "evolution of the translation system is envisaged to occur in a compartmentalized ensemble of replicating, co-selected RNA segments, i.e., in an RNA world containing ribozymes with versatile activities".

PRESENTATION OF THE HYPOTHESIS

We propose that coded protein synthesis arose from a noncoded process in an RNA world as a natural consequence of the accumulation of a range of early tRNAs and their serendipitous RNA binding partners. We propose that, initially, RNA molecules with 3' CCA termini that could be aminoacylated by ribozymes, together with an ancestral peptidyl transferase ribozyme, produced small peptides with random or repetitive sequences. Our concept is that the first tRNA arose in this context from the ligation of two RNA hairpins and could be similarly aminoacylated at its 3' end to become a substrate for peptidyl transfer catalyzed by the ancestral ribozyme. Within this RNA world we hypothesize that proto-mRNAs appeared first simply as serendipitous binding partners, forming complementary base pair interactions with the anticodon loops of tRNA pairs. Initially this may have enhanced stability of the paired tRNA molecules so they were held together in close proximity, better positioning the 3' CCA termini for peptidyl transfer and enhancing the rate of peptide synthesis. If there were a selective advantage for the ensemble through the peptide products synthesized, it would provide a natural pathway for the evolution of a coding system with the expansion of a cohort of different tRNAs and their binding partners. The whole process could have occurred quite unremarkably for such a profound acquisition.

TESTING THE HYPOTHESIS

It should be possible to test the different parts of our model using the isolated contemporary 50S ribosomal subunit initially, and then with RNAs transcribed in vitro together with a minimal set of ribosomal proteins that are required today to support protein synthesis.

IMPLICATIONS OF THE HYPOTHESIS

This model proposes that genetic coding arose de novo from complementary base pair interactions between tRNAs and single-stranded RNAs present in the immediate environment.

REVIEWERS

This article was reviewed by Eugene Koonin, Rob Knight and Berthold Kastner (nominated by Laura Landweber).

The role of aminoacyl-tRNA binding site on the factor EF-T in uncoupled GTPase reaction

The properties of the N-tosyl-L-phenylalanyl chloromethane treated EF-T factor were studied in a ribosomal system in which splitting of GTP occurs. The action of N-tosyl-L-phenylalanyl chloromethane inhibits the binding of aminoacyl-tRNA to the EF-T factor. The binding and exchange of guanosine phosphates continued to be preserved. The inhibited factor is inactive in the GTPase reaction which depends on the participation of ribosomes and aminoacyl-tRNA. The uncoupled GTPase reaction (which is not dependent on the presence of aminoacyl-tRNA) is also sensitive to the effect of the inhibitor. The inhibition of the uncoupled GTPase is incomplete. These findings are attributed to the involvement of the aminoacyl-tRNA binding site of the EF-T factor in the uncoupled GTPase reaction.

Differences in the ribosomes prepared from lactating and non-lactating bovine mammary gland

1. Ribosomes prepared from bovine lactating mammary gland are able to synthesize protein, whereas similar preparations from non-lactating glands are not. Washing the ribosome suspensions through a medium containing 0.5m-ammonium chloride enhanced their ability to incorporate phenylalanine into polyphenylalanine. 2. Ribosomes isolated from non-lactating bovine mammary gland, in contrast with those from rat liver and lactating mammary gland, contained significant amounts of extraneous nucleases. These enzymes could be removed by washing with a medium A buffer containing 0.5m-ammonium chloride. 3. Only those ribosomes from functionally active tissues were able to bind polyuridylic acid and phenylalanyl-tRNA.