Catalyzes the formation of 3-(3-amino-3-carboxypropyl)uridine (acp3U) at position 20a in the D-loop of several cytoplasmic tRNAs (acp3U(20a)) (PubMed:31804502). Also has a weak activity to form acp3U at position 20 in the D-loop of tRNAs (acp3U(20)) (PubMed:31804502).