The antisuppressor strain sin1 of Schizosaccharomyces pombe lacks the modification isopentenyladenosine in transfer RNA

Abstract

The modified base N 6-(Δ2-isopentenyl)-adenosine (i 6A) is missing in all transfer RNAs isolated from the antisuppressor strain sin1 of Schizosaccharomyces pombe. i 6A is found adjacent to the 3′ side of the anticodon of several tRNAs of S. pombe. Sequence analysis of tyrosine tRNA from the antisuppressor strain sin1 shows an unmodified adenosine instead of the i 6A. i 6A-deficient tyrosine tRNA elutes much earlier than wild-type tRNA Tyr during reverse phase chromatography (RPC-5). Serine tRNA and tryotophan tRNA from the sin1 mutant show a similar shift in the elution profile. We therefore conclude that these two tRNAs are also deficient in i 6A. The presence of the antisuppressor mutant sin1 leads to inactivation of the nonsense suppressor sup3-i. As sup3-i is a mutated serine tRNA, we conclude that the loss of the modification i 6A on the suppressor tRNA is responsible for the inactivation of sup3-i. Compared to wild type, the growth rate of the sin1 strain is only slightly reduced and the other i 6A-deficient tRNAs seem to function normally. We assume that the sin1 mutation affects the structural gene of an enzyme in the isopentenyl pathway, probably the transferase.