The reason why fucA1 is regulated by programmed À1 frameshifting is not known. However, the physiological significance of programmed frameshifting has been assigned to a minority of the cellular genes while for most of them it is still uncertain [see (4) and reference therein; (16) ]. This mechanism of recoding is exploited to set the ratio of two polypeptides such as the t and g subunits of the DNA polymerase III holoenzyme in E.coli (12) . Alternatively, programmed frameshifting balances the expression of a protein, as the bacterial translational release factor 2 and the eukaryotic ornithine decarboxylase antizyme [see (4) and (18) and references therein]. In the case of fucA1, the polypeptide encoded by the smaller ORF SSO11867 could never be detected by western blots analyses. In addition, the modelling of Ssa-fuc on the high-resolution crystal structure of the a-L-fucosidase from Thermotoga maritima (25, 37) showed that the fucA1 N-terminal polypeptide is not an independent domain. Moreover, we have shown recently that SSO11867 includes essential catalytic residues (27) , excluding the possibility that a functional a-fucosidase can be obtained from the ORF SSO3060 alone. Therefore, several lines of evidence allow us to exclude that programmed À1 frameshifting is used to set the ratio of two polypeptides of the a-fucosidase from S.solfataricus. More probably, this translational mechanism might be required to control the expression level of fucA1.