tRNAHis maturation:: An essential yeast protein catalyzes addition of a guanine nucleotide to the 5′ end of tRNAHis

All tRNA(His) molecules are unusual in having an extra 5' GMP residue (G(-1)) that, in eukaryotes, is added after transcription and RNase P cleavage. Incorporation of this G, residue is a rare example of nucleotide addition occurring at an RNA 5' end in a normal phosphodiester linkage. We show here that the essential Saccharomyces cerevisiae ORF YGR024c (THG1) is responsible for this guanylyltransferase reaction. Thg1p was identified by survey of a genomic collection of yeast GST-ORF fusion proteins for addition of [alpha-P-32]GTP to tRNA(His). End analysis confirms the presence of G(-1). Thg1p is required for tRNA(His) guanylylation in vivo, because cells depleted of Thg1p lack G(-1) in their tRNA(His). His(6)-Thg1p purified from Escherichia coli catalyzes the guanylyltransferase step of G(-1) addition using a ppp-tRNA(His) substrate, and appears to catalyze the activation step using p-tRNA(His) and ATP. Thg1p is highly conserved in eukaryotes, where G(-1) addition is necessary, and is not found in eubacteria, where G, is genome-encoded. Thus, Thg1p is the first member of a new family of enzymes that can catalyze phosphodiester bond formation at the 5' end of RNAs, formally in a 3'-5' direction. Surprisingly, despite its varied activities, Thg1p contains no recognizable catalytic or functional domains.