Previous experiments have shown that the frameshift cassette of mammalian antizyme 1 can direct ef®cient +1 frameshifting when tested in S.pombe. The reverse experiment was conducted here. The SPA gene was translated in vitro in rabbit reticulocyte lysate and its resulting frameshift ef®ciency measured. With no addition of polyamines, frameshifting ef®ciency is~1.5%. Addition of spermidine to the translation mixture to a ®nal concentration of 1 mM results in a 3.7-fold increase in frameshifting to~5.5%, a level even higher than that observed in the endogenous system in vivo (autoradiogram not shown). The observed ef®ciency of frameshifting with the SPA frameshifting cassette in vivo in S.pombe is signi®cantly more than that expected from its limited nucleotide similarity to the antizyme frameshift sites of higher eukaryotes. This prompted a search for additional stimulatory elements within the SPA frameshift cassette.