A breakdown of tRNAs by isoacceptor type and anticodon is given in Tables 3.3 and 3.4. Only four C. elegans tRNA species contain introns (Ile-UAU, Leu-CAA, Thr-UGU, Tyr-GUA), compared to 10 species in S. cerevisiae (Ile-UAU, Leu-CAA, Leu-UAG, Lys-UUU, Phe-GAA, Pro-UGG, Ser-GCU, Ser-CGA, Trp-CCA, Tyr-GUA). The total percentage of intron-containing tRNA genes in C. elegans is 5%, whereas 21% of S. cerevisiae tRNA genes contain introns. The role of introns within tRNAs has not been widely studied, although for several yeast tRNAs, an intron is necessary for correct addition of anticodon base modifications [Johnson & Abelson, 1983,Strobel & Abelson, 1986]. Clearly, the evolutionary pressures to increase or reduce introns within tRNAs is independent of those influencing the occurrence of mRNA introns, which occur at a low frequency in yeast relative to C. elegans.
tRNA species copy number in C. elegans ranged from 1 (Arg-CCG, Selenocysteine (SeC)-UCA) to 33 (Pro-UGG). tRNAs were somewhat evenly distributed among the chromosomes except for a disproportionate number on the X chromosome, which contained nearly half of all tRNAs in the genome (see Table 3.2).