prior to group 2 after the genetic code change, which is speculated on the basis of the following observations. It has been generally considered that reconstructed tRNA does not lose its activity during the several reaction First, the homology between tRNA Ser CAGs in group 1 and its isoacceptor tRNAs for codon UCG is higher than steps needed in the microsurgery method, such as cleavage of the tRNA strand and ligation of tRNA fragments that between the tRNA Ser CAG from C.cylindracea and its isoacceptor . Second, C.cylindracea (Ohyama et al., 1985) . However, a slight reduction of leucylation activity was observed in the control variant, (group 2) possesses high copy numbers of the tRNA Ser CAG genes (~20 copies) on the diploid genome (Suzuki z-G33G, compared with that of the native tRNA ( Figure  3A , right-hand graph), which turned out to result from et al., 1994) , while low copy numbers (two or four copies) are observed for group 1 tRNA Ser CAG genes (Santos the partial deacetylation of 4-acetyl cytidine (ac4C) due to acid treatment of the 5Ј-half fragment (see Materials et al., 1993; Sugiyama et al., 1995; T.Suzuki, personal observations) . Third, the codon CUG is utilized as a major and methods). Nevertheless, it is reasonable to deduce the effect of base replacement on the aminoacylation activity serine codon on several genes in C.cylindracea, such as lipase (Kawaguchi et al., 1989) and chitin synthase by comparing the activities of these reconstructed tRNAs, because the same 5Ј-half fragments were used for all the (unpublished results) , while CUG appears infrequently on the genomes of other species belonging to group 1 (Lloyd manipulated tRNA molecules of C.zeylanoides.