A carboxylesterase from the hyperthermophilic archaeon Sulfolobussolfataricus: cloning of the gene, characterization of the protein

Abstract

A genomic library of the hyperthermophilic archaeon Sulfolobussolfataricus strain MT4 was constructed in Escherichiacoli using a cloning vector not designed for heterologous gene expression. One positive clone exhibiting acquired thermophilic acetylesterase activity was directly detected by an insitu plate assay using a colony staining procedure with the chromogenic substrate β-naphthyl acetate. The plasmid isolated from the clone contained a 3.3 kb genomic fragment from S.solfataricus and a full-length esterase coding sequence could be identified. Expression of the active thermostable esterase in E.coli was independent of isopropyl-β-d-thiogalactopyranoside and of the kind of vector, suggesting that the archaeal esterase gene was controlled by fortuitous bacterial-like sequences present in its own 5′ flanking region, not by the bacterial lac promoter or other serendipitous vector-located sequences. The protein, partially purified by thermoprecipitation of the host proteins at high temperature and gel exclusion chromatography, showed a homo-tetrameric structure with a subunit of molecular mass of 32 kDa which was in perfect agreement with that deduced from the cloned gene. The same protein was revealed in S.solfataricus cell extracts, thus demonstrating its functional occurrence invivo under the cell culture conditions tested. The recombinant enzyme exhibited high thermal activity and thermostability with optimal activity between pH 6.5 and 7.0. The hydrolysis of p-nitrophenyl esters of fatty acids (from C2 to C8) allowed the enzyme to be classified as a short length acyl esterase.