LacS mutants of Sulfolobus solfataricus were studied. The mutants were isolated and characterized. The results showed that the mutants had a defect in the lacS gene, which is responsible for the transport of lactose. The lacS gene was cloned and sequenced, and the results were used to design a functional complementation assay. The assay was used to test the ability of the mutants to transport lactose. The results showed that the mutants were unable to transport lactose, and that the lacS gene was essential for this function. The study provides new insights into the biology of Sulfolobus solfataricus and the mechanisms of lactose transport in this organism.