Cytoplasmic non-canonical poly(A) RNA polymerase that catalyzes the transfer of one adenosine molecule from an ATP to an mRNA poly(A) tail bearing a 3'-OH terminal group and participates in the cytoplasmic polyadenylation (PubMed:33882302). Polyadenylates mRNA encoding extracellular matrix constituents and other genes crucial for bone mineralization and during osteoblast mineralization, mainly focuses on ER-targeted mRNAs (PubMed:33882302, PubMed:26803617). ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Expressed in primary osteoblasts derived from neonatal calvaria and osteocytes (PubMed:33882302). Strongly expressed in calvaria and femur (PubMed:26803617). Highly expressed in the dental sheet, oral epithelium, and mesenchyme at E12.5; in the tooth germ bud, ectomesenchyme and oral epithelium at 13.5; and in the dental papilla and dental epithelium at E14.5. Increased during osteoblast differentiation. Homozygous mice lacking Tent5a have slightly decreased survival, probably due to perinatal or embryonic lethality, since the effect is already visible at postnatal day 6 (P6). Mutant mice have an abnormal posture with frequent kyphosis and a wavy tail. Mutant mice exhibit frequent bone fractures and newborn mice reveal rib fractures. Adult mutant mice show decreased cartilage ossification of the tail. Belongs to the TENT family.