Polyamines play important roles in cell growth mainly through their interaction with RNA. We have previously reported that polyamines stimulate the synthesis of oligopeptide-binding protein OppA in Escherichia coli and the formation of Ile-tRNA in rat liver (Igarashi, K., and Kashiwagi, K. (2000) Biochem. Biophys. Res. Commun. 271, 559-564). These effects involve an interaction of polyamines with the bulged-out region of double-stranded RNA in the initiation region of OppA mRNA and in the acceptor stem of rat liver tRNA(Ile). In this study, the effects of polyamines on E. coli OppA synthesis and rat liver Ile-tRNA formation were compared using OppA mRNA and tRNA(Ile) with or without the bulged-out region of double-stranded RNA. The results indicate that the bulged-out region is involved in polyamine stimulation of OppA synthesis and Ile-tRNA formation. A selective structural change by spermidine in the bulged-out region of double-stranded RNA was confirmed by circular dichroism.