Probably involved in an RNA processing event other than mRNA capping. Releases gamma-phosphate from the 5'-end of RNA to produce a diphosphate terminus. a 5'-end triphospho-(purine-ribonucleoside) in mRNA + H2O = a 5'-end diphospho-(purine-ribonucleoside) in mRNA + H(+) + phosphate Magnesium and/or manganese. Specific for polynucleotide RNA in the presence of magnesium, but becomes specific for nucleotide triphosphates in the presence of manganese. Belongs to the fungal TPase family.