Isolation and Characterization of the TRMl Locus , a Gene Essential for the N 2 , N 2 - Dimethylguanosine Modification of Both Mitochondrial and Cytoplasmic tRNA in Saccharomyces cereuisiae ”

The trml mutation of Saccharomyces cerevisiae is a single nuclear mutation that affects a specific base modification of both cytoplasmic and mitochondrial tRNA. Transfer RNA isolated from trml cells lacks the modified base N2,N2-dimethylguanosine, and extracts from these cells do not have detectable N2,NZdimethylguanosine-specific tRNA methyltransferase activity. As part of our efforts to determine how this mutation affects enzyme activities in two different cellular compartments we have isolated the TRMl locus by genetic complementation. The TRMl locus restores the N2,N2-dimethylguanosine modification to both cytoplasmic and mitochondrial tRNA in trml cells. An open reading frame in this TRMl gene is essential for complementation of the trml phenotype. Expression of this open reading frame in Escherichia coli converts the organism from one that neither makes N2,N2-dimethylguanosine nor has N2,N2-dimethylguanosine-specific tRNA methyltransferase activity into one that does. This result suggests that the TRMl locus is the structural gene for the tRNA modification enzyme and that both nuclear/cytoplasmic and mitochondrial forms of the methyltransferase are produced from the same gene.