Mechanism of suppression in Drosophila. VII. Correlation between disappearance of an isoacceptor of tyrosine tRNA and activation of the vermilion locus.

The possibility that tyrosine tRNA modifies the catalytic activity of tryptophan oxygenase that is produced by the vermilion mutant (v) in Drosophila melanogaster is reconsidered. Dietary conditions can modify the ratio of the two major isoacceptors of tyrosine tRNA: one condition allows 85--90% to exist as the second isoacceptor, and another condition allows less than 5% to exist in this form. The function lacking in the vermilion mutant is partially restored when the second isoacceptor of tRNATyr is reduced to low levels (less than 40%), but the function is greatly reduced when this isoacceptor is present as 50% or more of the total. These data support the hypothesis that tRNATyr may be associated with and regulate tryptophan oxygenase. The corresponding isoacceptor of tRNATyr found in a suppressor mutant, su(s)2, should not have any effect on the function of the vermilion gene, and, indeed, it did not. The tRNAs for tyrosine, aspartic acid, and histidine all have one isoacceptor that contains nucleoside Q and all undergo parallel changes in flies raised on the various diets. It appears that these dietary changes affect the ability to synthesize or modify Q or to remove or insert it into tRNA.