Biosynthesis of Eukaryotic Transfer RNA

Abstract

Each eukaryotic cell contains about 100 different transfer RNA species, which are polynucleotides whose chainlength varies from 73 to 93 (Sprinzl et al. 1980). Transfer RNAs play a key role in protein biosynthesis: They carry the amino acids to the ribosome, where they pair with messenger RNA to ensure the correct placement of each amino acid into the growing polypeptide chain. The structural features of tRNA, therefore, must contain all the information for specific interactions with many components in the protein-synthesizing machinery. The number of specific interactions of tRNA is even greater when one considers the complex sequence of enzymatic steps involved in the biosynthesis of these molecules (Figure 1). The tRNA genes are transcribed into larger precursor RNA molecules, which are subsequently cleaved to mature-size tRNA by the action of special processing nucleases. Another part of this maturation process is the synthesis of modified nucleosides, which proceeds as posttranscriptional nucleotide modification. The study of tRNA biosynthesis is very difficult since the steady-state level of tRNA precursors in normal cells is low. The biochemical approach to this problem was made possible by recent methodological developments in recombinant DNA research and DNA sequence analysis. Insight into the structure of tRNA genes, coding for a variety of tRNAs, allowed the search for conserved sequences and attempts to define transcription regulatory signals in the DNA. The use of tRNA genes with known nucleotide sequence led to the characterization of enzymatic systems, which accurately transcribed such genes into tRNA precursors. These RNA species can then be used to unravel the