Catalyzes the transfer of a dimethylallyl group onto the adenine at position 37 in tRNAs that read codons beginning with uridine, leading to the formation of N6-(dimethylallyl)adenosine (i(6)A). adenosine(37) in tRNA + dimethylallyl diphosphate = diphosphate + N(6)-dimethylallyladenosine(37) in tRNA Strongly inhibited by ADP, ATP and other NTPs. Optimum pH is 6.5-9. Monomer. Binds to tRNA(Phe). By 2-aminopurine. Specific tRNA undermodification may be a switch to higher mutation frequency when cells are subject to environmental stresses other than treatment with mutagens. Belongs to the IPP transferase family.