Drosophila tRNA can be guanylated by a crude enzyme from rabbit reticulocytes. Guanylating activity is also present in crude extracts of adult Drosophila A major product of this reaction as well as several minor ones were resolved by RPC-5 chromatography. The main substrate of both the Drosophila and rabbit reticulocyte enzymes was the non-Q-containing aspartic acid tRNA, tRNAAsp. The Q-lacking (y) forms of asparagine, histidine and tyrosine tRNAs were also substrates and gave rise to the minor products of the reaction. In contrast, the Q- or Q*-containing (δ) forms of these tRNAs appear not to be substrates. The evidence strongly suggests that the guanyating enzyme is involved in Q biosynthesis and would be better termed a guanine replacement or pre-Q insertion enzyme.
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