The archaeal Sulfolobus solfataricus elongation factor 1a (SsEF-1a) bound to GTP or to its analogue guanyl-5'-yl imido diphosphate [Gpp(NH)p] formed a ternary complex with either Escherichia coli Val-tRNAVal or Saccharomyces cerevisiae Phe-tRNAPhe as demonstrated by gel-shift and gel-filtration experiments. Evidence of such an interaction also came from the observation that SsEF-1a-Gpp(NH)p was able to display a protective effect against either the spontaneous deacylation or the digestion of aminoacyl-tRNA by RNase A. Protection against the deacylation of aminoacyl-tRNA allowed evaluatation of the affinity of SsEF-1a-Gpp(NH)p for both aminoacyl-tRNAs used. The K'd values of the ternary complex containing S. cerevisiae Phe-tRNAPhe or E. coli Val-tRNAVal were 0.3 mm and 4.4 mm, respectively. In both cases, the affinity of SsEF-1a-Gpp(NH)p for aminoacyl-tRNAwas three orders of magnitude lower than that of the homologous eubacterial ternary complexes, but comparable with the affinity shown by the ternary complex involving eukaryal EF-1a [Negrutskii, B.S. & El'skaya, A.V. (1998) Prog. Nucleic Acids Res. 60, 47±77]. As already observed with eukaryal EF-1a, SsEF-1a in its GDP-bound form was also able to protect the ester bond of aminoacyl-tRNA, even though with a 10-fold lower efficiency compared with SsEF-1aXGpp(NH)p. The overall results indicated that the archaeal elongation factor 1a shares several properties with eukaryal EF-1a but not with eubacterial EF-Tu.
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