A genomic region upstream of the alcohol dehydrogenase (Ssadh) gene was cloned and sequenced from a library of Sulfolobus solfataricus MT4 strain. The isolated 4,040-bp DNA fragment revealed an open reading frame (celS), lying in the opposite direction to Ssadh, which showed significant similarity to endo-beta-1,4-glucanases from Pyrococcus furiosus, Thermotoga maritima, and Thermotoga neapolitana. celS was shown to be a functional gene in vivo: a specific celS mRNA was detected by primer extension analysis showing a unique initiation transcription site coinciding with the ATG translation initiation codon. The specific gene product was detected as an extracellular cellulase after enzyme staining by carboxymethyl cellulose (CMC) SDS-PAGE, showing a molecular weight in agreement with that deduced from the open reading frame. Depending on growth conditions, different levels of cellulase activity and specific celS transcript were detected, revealing an inductive effect of CMC and suggesting a repressive role of glucose.