In this study, we have analysed the features of mRNA/ribosome interaction in the thermophilic archeon Sulfolobus solfataricus. Leadered mRNAs endowed with ShineDalgarno (SD) motifs formed stable binary complexes with 30S subunits, optimally at high temperature (6570 degrees C) and without the aid of initiator tRNA (tRNAi) or any factor. 'Toeprinting' assays revealed that the SD motifs were necessary and sufficient to direct the 30S subunit to the translation initiation region. Leaderless mRNAs, i.e. mRNAs entirely lacking a 5'-untranslated region (UTR), did not interact directly with 30S subunits but required the presence of tRNAi, indicating that codonanticodon pairing was required for positioning the ribosome on the initiation codon. The data suggest that archaea such as Sulfolobus routinely use two distinct mechanisms for translational initiation. SD-dependent initiation, resembling the pathway prevalent in present-day bacteria, would operate on distal cistrons of polycistronic mRNAs, whereas 'leaderless' initiation, reminiscent of the eukaryotic pathway, would operate on monocistronic mRNAs and on opening cistrons of polycistronic mRNAs.