We have combined Cotten and Birnstiel's tRNA-embedded ribozymes and our 5'- and 3'-trimming system. Although the activity of the tRNA-embedded ribozyme was ca. 30% lower than those of naked ribozymes, since the stability of the former in bovine serum was higher than those of naked ribozymes, the tRNA-embedded ribozymes appear useful especially when the 5'- and 3'-trimming units are concatenated in tandem.