Tryptophan tRNA isolated from a UGA suppressor (SuUGA+) strain of Escherichia coli, CAJ64, has been partially purified on a benzoylated DEAE-cellulose column. When added to an in vitro system this tRNA was able to suppress a UGA mutation in the phage-specific polymerase of a mutant of bacteriophage f2 (op 9). Tryptophan tRNA obtained from an Su-strain did not cause suppression, nor did tRNA from the SuUGA+strain which had undergone a similar purification process but was enriched for tyrosine or phenylalanine acceptor activity. The suppressing tRNA was further fractionated and the suppressor activity correlated with tryptophan acceptor activity. We conclude that the suppressor tRNA species in E. coli CAJ64 is a tryptophan tRNA. The UGA mutant is known to be leaky. In order to facilitate the suppressor assay, use was made of an extract prepared from an Su-strrstrain in which residual synthesis of polymerase stimulated by op 9 RNA was markedly reduced. We have demonstrated by mixing experiments that it is the ribosome fraction in the strrextract which is responsible for the observed streptomycin effect. © 1971.
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