Summary: Acetylesterase and cinnamoyl ester hydrolase activities were demonstrated in culture supernatant of the anaerobic ruminal fungus A cDNA expression library from was screened for esterases using β-naphthyl acetate and a model cinnamoyl ester compound. cDNA clones representing four different esterase genes () were isolated. None of the enzymes had cinnamoyl ester hydrolase activity, but two of the enzymes (BnaA and BnaC) had acetylxylan esterase activity. and encode proteins with several distinct domains. Carboxy-terminal repeats in BnaA and BnaC are homologous to protein-docking domains in other enzymes from species and another anaerobic fungue, a sp. The catalytic domains of BnaB and BnaC are members of a recently described family of Ser/His active site hydrolases [Upton, C. & Buckley, J. T. (1995). 20, 178-179]. BnaB exhibits 40% amino acid identity to a domain of unknown function in the CeIE cellulase from and BnaC exhibits 52% amino acid identity to a domain of unknown function in the XynB xylanase from BnaA, whilst exhibiting less than 10% overall amino acid identity to BnaB or BnaC, or to any other known protein, appears to be a member of the same family of hydrolases, having the three universally conserved amino acid sequence motifs. Several other previously described esterases are also shown to be members of this family, including a rhamnogalacturonan acetylesterase from However, none of the other previously described enzymes with acetylxylan esterase activity are members of this family of hydrolases.


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