Histidine tRNA guanylyltransferase from Saccharomyces cerevisiae. I. Purification and physical properties

Compared to other tRNAs all known histidine tRNAs have the unique feature of possessing an additional nucleotide at their 5' end. It is usually a guanosine residue but not in bacteriophage T5 tRNA which carries an additional uridine. The additional nucleotide is not encoded in eukaryotic histidine tRNA genes but is added in a post-transcriptional modification reaction (Cooley, L., Appel, B., and Söll, D. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 6475-6479) by histidine tRNA guanylyltransferase (tRNAHis guanylyltransferase). Here we report the purification of this enzyme from Saccharomyces cerevisiae and the determination of some of its physical properties. Six different steps including Polymin P precipitation, chromatography on DEAE-cellulose, phosphocellulose, heparin-agarose, ATP-agarose, and gel filtration on Superose 12 were employed for the purification of the guanylyltransferase from an S-100 extract. A Stokes radius of 46.5 +/- 0.5 A and a sedimentation coefficient (S20,w) of 7.8 +/- 0.2 were determined by gel filtration and rate zonal sedimentation, respectively. A relative molecular weight (Mr) of approximately 120,000 was calculated for the purified native enzyme. The Mr of the denatured protein is approximately 58,000 as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These results indicate a homodimeric (alpha 2) structure for the enzyme. Among all acceptor RNAs in unfractionated tRNA only tRNAHis is a substrate for the purified guanylyltransferase. The reaction requires ATP, a guanosine substrate, and a divalent metal ion. Treatment of guanylyltransferase with 5,5'-dithiobis(2-nitrobenzoic)-acid abolishes activity; this suggests the importance of sulfhydryl groups for enzymatic activity. The enzyme shows discrimination among different histidine tRNA species; tRNA from plant and prokaryotes are better substrates than mammalian and insect tRNAs.