Isopentenyladenosine deficient tRNA from an antisuppressor mutant of Saccharomyces cerevisiae.

We have isolated a mutant of Saccharomyces cerevisiae that contains 1.5% of the normal tRNA complement of isopentenyladenosine (i6A). The mutant was characterized by the reduction in efficiency of a tyrosine inserting UAA nonsense suppressor. The chromatographic profiles of tRNATyr and tRNASer on benzoylated DEAE-cellulose are consistent with the loss of i6A by these species. Transfer RNA from the mutant exhibits 6.5% of the cytokinin biological activity expected for yeast tRNA. Transfer RNAs from the mutant that normally contain i6A accept the same levels of amino acids in vitro as the fully modified species. With the exception of i6A, the level of modified bases in unfractionated tRNA from the mutant appears to be normal. The loss of i6A apparently affects tRNA's role in protein synthesis at a step subsequent to aminoacylation.