Catalyzes the formation of 3-(3-amino-3-carboxypropyl)uridine (acp3U) at position 20 in the D-loop of several cytoplasmic tRNAs (acp3U(20)). a uridine in tRNA + S-adenosyl-L-methionine = a 3-[(3S)-3-amino-3-carboxypropyl]uridine in tRNA + H(+) + S-methyl-5'-thioadenosine Contains 1 DXTW motif. Belongs to the TDD superfamily. DTWD1 family.