Aqualysin I is an alkaline serine protease which is secretet into the culture medium by Thermos aquaticus YT-1, an extreme thermophile. Aqualysin I was purified, and its partial amino acid sequence was determined. The gene encoding aqualysin I was cloned into E. coli using synthetic oligodeoxyribonucleotides as hybridization probes. The nucleotide sequence of the cloned DNA was determined. The primary structure of aqualysin I, deduced from the nucleotide sequence, agreed with the determid amino acid sequences, including the NH₂- and COOH terminal sequence of the tryptides derived from aqualysin I. Aqualysin I comprised 281 amino acid residues and its molecular mass was determined to 1)e 28350. On alignment of the whole amino acid sequence, aqualysin I showed high sequence homology with the subtilisin type serine protease, and 43% identity with proteinase K, 37-30% with subtilisins and 34% with thermitase. Extremely high sequence identity was observed in the regions containing the active-site residues, corresponding to Asp32, His64 and Ser221 of subtilisin BPN`. Aqualysin I contains two disulfide bonds, Cys67-Cys99 and Cys163-Cysl94, and these disulfide bonds seem to contribute to the heat stability of the enzyme. The determined positions of the twe disulfide bonds of aqualysin I agreed with those predicted previously on the basis of computer graphics of the crystallographic data for subtilisin BPN`. Therefore, these findings sugests that the three-dimensional structure of aqualysin I is similar to that of subtilisin BPN` Aqualysin I is produced as a lage precursor, which contains NH₂- and COOH- terminal portions besides the mature protease sequence.