Role of Modified Nucleosides in Transfer RNA

Abstract

<p>Transfer ribonucleic acids are indispensable components of cellular protein synthesising systems. Besides the four major nucleosides namely, uridine, cytidine, adenosine and guanosine, tRNA molecules contain a number of modified nucleosides. An attempt is made in this thesis to investigate the functional roles of two such modified nucleosides present in tRNAs from baker's yeast.</p> <p>Two species of lysine accepting tRNA and one species of each of isoleucine, threonine, arginine and asparagine accepting tRNAs were purified from baker yeast. All of these tRNAs recognised codons starting with adenosine. A fast and sensitive method using high pressure cation exchange chromatography, was developed for analysis of the modified nucleosides present in these tRNAs. The modified nucleoside, N-[9(β-D-ribofuranosyl)purin-6-vlcarbamoyl] threonine (t⁶A) was present in each of the above mentioned species of tRNA. The data provided further support to the generalisation that all tRNAs recognising cations starting with the adenosine contained t⁶A.</p> <p>Another modified nucleoside 2-thio-5-carboxymethyluridine (U*) was present only in one species of lysine tRNA (tRNA₂^(Lys). U* was known to be present at the 5' end of the anticodon in a few tRNAs which only recognised codon XYA. The coding properties of tRNA₁^(Lys) and tRNA₂^(Lys) were studied to investigate the role or U* in tRNA₂^(Lys). Further studies in this line were performed by chemically modifying U* in tRNA₂^(Lys) and observing its effect on the coding properties of tRNA₂^(Lys). These studies revealed that U* not only influenced the coding properties of tRNA₂^(Lys) but also was important in maintaining the efficiency of tRNA₂^(Lys) in the process of in vitro protein synthesis.</p>