Studies of a Methyltransferase and an Exonuclease Involved in Initiator tRNAmet Maturation in Saccharomyces Cerevisiae

More than 95% of the nucleic acid found in yeast cells is non-coding ribonucleic acid (RNA), which is RNA that specifies a RNA product, rather than a protein product (1). Noncoding RNA in eukaryotes includes transfer RNA (tRNA), small nuclear RNA (snRNA), small nucleolar RNA (snoRNA), and ribosomal RNA (rRNA), which is the most abundant noncoding RNA (2). Formation of a functional noncoding RNA does not result from the transcription of the gene alone---a number of processing and modification reactions occur before maturation. For example, in the case ofrRNA, maturation involves cleavage of polycistronic transcripts to release individual rRNAs, trimming of 3\u27 ends, and modification of nucleotides (3). Likewise, tRNAs are transcribed as precursors with extra nucleotides at the 5\u27 and 3\u27 ends that need to be eliminated (4) (Fig. 1.1). In addition, many nucleotides are modified, introns are removed, and the sequence CCA is added to the 3\u27 end in preparation for aminoacylation (Fig. 1.1). In general, synthesis of noncoding RNAs is complex and requires that cells not only transcribe the RNAs, but also manufacture additional RNAs, proteins, and cofactors that are involved in producing the mature RNA. I have used the yeast Saccharomyces cerevisiae to study processing and modification events in the production of tRNA, the noncoding RNA that is essential for messenger RNA to be translated and proteins produced. tRNA was discovered in the late 1950\u27s when it was found that radiolabeled leucine could be transferred to an RNA and then into a protein (5). The first tRNAs to be purified in significant amounts were yeast tRNAs, and, as a result, the first tRNA to be sequenced was yeast tRNA Ala (6).g.