In vitro enzymatic methylation of TMV RNA

Evidence that RNA 4 of alfalfa mosaic virus does not replicate autonomously

A mutant (Tbts7) of alfalfa mosaic virus, the coat protein of which is unable to activate the viral genome (the RNA species 1, 2, and 3, which need some coat protein for infectivity) at 30 degrees , can be rescued at this temperature by adding to the inoculum wild-type RNA 3 (the genome part that contains the coat protein cistron), but not adding wild-type RNA 4 (the subgenomic messenger for the coat protein). Unless RNA 3 of Tbts 7 has a second ts mutation at a site not occurring in RNA 4, it may be concluded from the above finding that RNA 4 does not replicate autonomously.

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.