Abstract
tRNA-guanine transglycosylase replaces a base in tRNA with guanine and uses free guanine as the substrate. This enzyme was obtained from adult Drosphila melanogaster and characterized as follows: K m for guanine is 2.8 × 10 −7 M; pH optimum is 7.4. The enzyme activity is present in third instar larvae and adult flies but is absent in late pupae. When Drosphila tRNAs were guanylated with the comparable enzyme from rabbit reticulocytes, at least four species of tRNA could be demonstrated to have incorporated radioactive guanine. The role for this enzyme appears to be to exchange queuine into tRNA and thus replace a guanine in the anticodon of tRNAs for Asn, Asp, His, and Tyr. In the absence of queuine it will catalyze the exchange reaction described above.
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