Modification of the 5' end of mRNA. Association of RNA triphosphatase with the RNA guanylyltransferase-RNA (guanine-7-)methyltransferase complex from vaccinia virus.

Purified RNA guanylyltransferase . RNA (guanine-7-)methyltransferase complex from vaccinia virus contains RNA triphosphatase activity. The latter activity, which removes the gamma-phosphate from triphosphate-ended polyribonucleotides, co-chromatographed with the capping and methylating enzyme complex on seven different ion exchange or affinity columns and co-sedimented with the complex on a glycerol gradient. On a molar basis, the RNA triphosphatase was about 100 times more active than the associated RNA guanylyl-transferase. When the purified enzyme complex was incubated with poly(A) containing a 5'-triphosphate, removal of the gamma-phosphate preceded capping. Furthermore, there was no significant difference in the rate or extent of capping 5'-diphosphate- or 5'-triphosphate-ended poly(A). Physical association of the three enzymatic activities appears to be an efficient mechanism for carrying out the following successive steps in cap formation: (formula: see text).