The functionality of the product of the mutant gene fucA1 A does not provide direct experimental evidence that programmed À1 frameshifting occurs in vivo and in S.solfataricus. To address these issues, we report here the study of the expression of the wild-type split gene fucA1 and of its mutants in the slippery sequence. We demonstrate here that fucA1 is expressed by programmed À1 frameshifting in both E.coli and S.solfataricus. This is the first experimental demonstration that this kind of recoding is present in the Archaea domain of life. The relevance of programmed À1 frameshifting in Archaea is also discussed. Figure 1 . The a-fucosidase gene. (A) Region of overlap in the wild-type split fucA1 gene. The N-terminal SSO11867 ORF is in the zero frame, the C-terminal SSO3060 ORF, for which only a fragment is shown, is in the À1 frame. The slippery heptameric sequence is underlined; the rare codons are boxed and the arrows indicate the stems of the putative mRNA secondary structure. The amino acids involved in the programmed À1 frameshifting and the first codon translated after this event in the À1 frame are shadowed. (B) Fragment of the full-length mutant fucA1 A gene. The small arrows indicate the mutated nucleotides.