Polymerase that creates the 3'-poly(A) tail of mRNA's. Also required for the endoribonucleolytic cleavage reaction at some polyadenylation sites. May acquire specificity through interaction with a cleavage and polyadenylation specificity factor (CPSF) at its C- terminus. . This is the function of ENSG00000090060 (PAPOLA, poly(A) polymerase alpha).