Mechanism of suppression in Drosophila: II. Enzymatic discrimination of wild-type and suppressor tyrosine transfer RNA

Abstract

Previous studies had shown that two principle forms of tyrosine transfer RNA of Drosophila melanogaster were present in wild-type adult flies but that the second form was virtually absent in a suppressor mutant, su(s) 2 . Current results are at variance with the previous ones, in that the suppressor mutant has significant amounts of the second form of tRNA Tyr. A second chromatography system for separating these forms of tRNA Tyr is described, RPC-5, and is compared to the system used previously, RPC-2. Both systems indicate that wild-type flies contain the two forms of tRNA Tyr in a ratio of 40 60 , the suppressor mutant in a ratio of 60 40 . The difference between current and previous results can be attributed to the procedures used in the preparation of the enzyme that is used as a source of tyrosyl-tRNA ligase. The enzyme activity can be separated into two fractions on DEAE-cellulose chromatography. With suppressor tRNA as substrate, one enzyme fraction charges both forms of tRNA Tyr but the second enzyme fraction charges the first form preferentially or nearly exclusively in some cases, as was seen in the previous experiments. With wild-type tRNA as substrate both enzyme fractions charge both forms of tRNA Tyr. Storage results in the loss of the enzyme's ability to discriminate against the second form of tRNA Tyr from the suppressor mutant, while the enzymatic activity is retained. We postulate that the su(s) + locus produces an enzyme that modifies the second isoacceptor of tRNA Tyr and that, when such modification fails to occur (as in the su(s) 2 mutant), the tRNA is unable to accept tyrosine from one form of tyrosyl-tRNA ligase. How the discrimination against the second isoacceptor by the ligase may be important metabolically is not apparent.