5' Cap methylation of homologous poly A(+) RNA by a RNA (guanine-7) methyltransferase from Neurospora crassa

Methylated cap structures in eukaryotic RNAs: structure, synthesis and functions

There are more than twenty capped small nuclear RNAs characterized in eukaryotic cells. All the capped RNAs appear to be involved in the processing of other nuclear premessenger or preribosomal RNAs. These RNAs contain either trimethylguanosine (TMG) cap structure or methylated gamma phosphate (Mppp) cap structure. The TMG capped RNAs are capped with M7G during transcription by RNA polymerase II and trimethylated further post-transcriptionally. The Mppp-capped RNAs are transcribed by RNA polymerase III and also capped post-transcriptionally. The cap structures improve the stability of the RNAs and in some cases TMG cap is required for transport of the ribonucleoproteins from cytoplasm to the nucleus. Where tested, the cap structures were not essential for their function in processing other RNAs.

In vitro methylation of undermethylated yeast poly(A)-rich RNA using mRNA(guanine-7-)-methyltransferase purified from wheat germ or yeast

By crossing two strains of Saccharomyces cerevisiae deficient for each of the two methionine adenosyltransferase isoenzymes (ATP: L-methionine S-adenosyltransferase EC 2.5.1.6) respectively, we have constructed a strain strictly auxotrophic for S-adenosylmethionine and used it as a source of undermethylated mRNA suitable for in vitro transmethylation studies. RNA has been phenol-extracted from yeast cells shifted down to S-adenosylmethionine-free medium for 90 min and poly(A)-rich RNA has been prepared by oligo(dT)-cellulose chromatography. Upon incubation in vitro in the presence of methyl-labeled S-adenosylmethionine and mRNA (guanine-7-)-methyltransferase purified from wheat germ or yeast, undermethylated poly(A)-rich RNA became significantly labeled as compared to non-starved cells from the same strain, or from a wild-type control. Cap structures were resolved by paper chromatography afer T2 and P1 RNase digestion, and shown to be a mixture of m7G5'ppp5'G and m7G5'ppp5'A, irrespective of the enzyme source, in agreement with earlier in vivo studies in yeast mRNA capping and methylation.

Purification and characterization of the messenger ribonucleic acid capping enzyme GTP:RNA guanylyltransferase from wheat germ

A GTP:RNA guanylyltransferase or capping enzyme has been purified approximately 2000-fold from wheat germ. The enzyme catalyzes the transfer of the GMP residue from GTP to the 5' end of RNA or synthetic polyribonucleotides. Diphosphate-ended polymers were capped more efficiently than molecules with triphosphate ends, and molecules with monophosphate ends were not capped at all. There appears to be little specificity since RNAs with purine or pyrimidine ends served as acceptors. Other features of the wheat germ RNA guanylyltransferase include relatively low Km values for GTP (2.7 microM) and ppA (pA)n (14.2 nM), a divalent cation requirement satisfied by low (0.5 mM) concentrations of MnCl2 or higher (5 mM) concentrations of MgCl2, and a pH optimum around neutrality.