Catalyzes the transfer of one adenosine molecule from an ATP to an mRNA poly(A) tail bearing a 3'-OH terminal group and enhances mRNA stability and gene expression (PubMed:32009146, PubMed:28931820, PubMed:34048638). Can also elongate RNA oligos ending with uridine molecule, provided that the sequence is adenosine-rich (PubMed:34048638). Mainly targets mRNAs encoding endoplasmic reticulum-targeted protein (PubMed:28931820). (Microbial infection) Seems to enhance replication of some viruses, including yellow fever virus, in response to type I interferon. ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Interacts with BCCIP and PABPC1; the interaction has no effect on TENT5C poly(A) polymerase function (PubMed:28931820). Interacts with PLK4; this interaction leads to the TENT5C recruitment into the centrosome (PubMed:32433990). Recruited into the centrosome through its interaction with PLK4. By type I interferons. Belongs to the TENT family.