Catalysis of the reaction: nucleoside triphosphate + RNA(n) = diphosphate + RNA(n+1); the synthesis of RNA from ribonucleotide triphosphates in the presence of a nucleic acid template, via extension of the 5'-end.
Source:PMID:27484477,
PMID:30917604,
PMID:22456265,
GOC:pf