A gene coding for an esterase (SshEstI, 915 bp in length) of the thermoacidophilic archaeon Sulfolobus shibatae DSM5389 was cloned, sequenced, and overexpressed in Escherichia coli JM109 cells as a soluble, catalytically active protein. The deduced amino acid sequence of SshEstI was consistent with a protein containing 305 amino acid residues with a molecular mass of 33 kDa. Sequence comparison studies indicated that SshEstI could be a member of the hormone-sensitive lipase family, in that it had the highest sequence similarity to esterases from Sulfolobus solfataricus (90% identity) and Archaeoglobus fulgidus (42%) and a lipase from Pseudomonas sp. B11-1 (38%). The recombinant enzyme was highly thermostable and retained more than 70% of its initial activity after incubation at 90degreesC and pH 7.0 for 30 min. The recombinant enzyme catalyzed the hydrolysis of p-nitrophenyl (p-NP) esters with C-2-C-16 acyl chains but not the hydrolysis of triacylglycerides such as tributyrin and triolein. The enzymatic hydrolysis of p-NP acetate proceeded in a linear manner with time, whereas that of p-NP esters with acyl chains of C, or longer showed a biphasic profile, where a rapid release of p-nitrophenol (similar to3 min) was followed by a slow, sustained release. These non-linear kinetics may be explained in terms of a very slow, presteady-state burst phenomenon of p-nitrophenol release or a hysteretic behavior of SshEstI with these substrates.