and C.cylindracea (Yokogawa et al., 1992; Ohama et al., 1993 ). The numbering system and abbreviations for modified nucleotides conform Candida zeylanoides tRNA Ser CAG is leucylated to Sprinzl et al. (1996) and Crain and McCloskey (1996), respectively. in vitro (B) Time-dependent aminoacylation with SerRS or LeuRS from First the leucylation of tRNA Ser CAGs from C.zeylanoides C.zeylanoides cells. Aminoacylation reactions were carried out with and C.cylindracea was examined using LeuRS partially 0.7 µM tRNAs and with same amounts of enzyme activities calculated using cognate tRNAs. Serylation and leucylation are shown by dotted purified from C.zeylanoides, since it is known that leucine and solid lines, respectively. The right-hand frame shows the solid tRNAs of yeast have one of their identity determinants at curves from left-hand frame plotted with an enlarged ordinate. The position 37 (Soma et al., 1996) and tRNA Ser CAGs of aminoacylation of C.zeylanoides tRNA Ser CAG (s) and of C.zeylanoides and C.cylindracea have different nucleo-C.cylindracea tRNA Ser CAG (u) are compared; C.cylindracea tRNA Ser GCU (j), having no leucylation activity, is shown as a tides at this position (m 1 G and A, respectively) (Figure control. (C) TLC analysis of acetylleucyl-tRNA fragments derived 1A). Both tRNAs showed almost full serylation activity from leucylated tRNA Ser CAGs. After leucylation with [ 14 C]leucine, (~1200-1500 pmol/A 260 unit), as shown in Figure 1B . The leucyl-tRNAs were acetylated with acetic anhydride. Acetyl-tRNA Ser CAG of C.zeylanoides was evidently leucylated leucylated at all, as was the case when another species tRNA Ser CAG by gel-electrophoresis under acidic con- observed with LeuRSs from both C.cylindracea and S.cerevisiae (data not shown). of serine tRNA specific for codon AGY (Y: U or C) m 1 G37 is responsible for recognition by (tRNA Ser GCU) was employed as a control substrate leucyl-tRNA synthetase ( Figure 1B , right-hand graph). The K m value of C.zeylan-Among the tRNA Ser CAGs of several Candida species, oides LeuRS towards tRNA Ser CAG (5.0 µM) is only one that of C.cylindracea is unique because it alone possesses order of magnitude larger than that of the serylation of no leucylation capacity. A sequence comparison of these this tRNA (0.22 µM) as well as that of leucylation toward tRNAs ( Figure 1A ) prompts us to speculate that the the cognate leucine tRNAs of S.cerevisae (0.34 µM; Soma nucleotide at position 37 is strongly associated with et al., 1996) .