Remarkably, under the same conditions at which fucA1 drives the expression of the full-length protein, we could not observe any product from the fucA1 sm and fucA1 tm constructs. These data demonstrate that the integrity of the heptanucleotide is essential for the expression of the fucA1 gene in S.solfataricus, thus further confirming that the gene is decoded by programmed À1 frameshifting in this organism. In addition, the lack of expression of fucA1 sm by translation in vitro in S.solfataricus contrasts with the efficient expression of this mutant in E.coli, indicating that the two organisms recognize different sequences regulating the translational frameshifting.