Nucleotide sequence of the Clostridium thermocellum bglB gene encoding thermostable β-glucosidase B: Homology to fungal β-glucosidases

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Abstract
The nucleotide sequence of the bglB gene, coding for the thermostable β-glucosidase B of Clostridium thermocellum was determined. The coding region of 2265 bp was identified by comparison with the N-terminal amino acid sequence of β-glucosidase B purified from Escherichia coli. The derived amino acid sequence corresponding to a polypeptide of Mr 84100 was confirmed by sequencing of the C-terminal peptide generated by cleavage with cyanogen bromide. The protein bears no resemblance to other bacterial β-glucosidase sequences. However, extensive regions of homology were identified between the C. thermocellum enzyme and fungal β-glucosidases. The N-terminal homologous region contains an amino acid sequence very similar to the active site of β-glucosidase A3 from Aspergillus wentii. The striking sequence similarities between C. thermocellum β-glucosidase B and Kluyveromyces fragilis β-glucosidase suggest the possibility of a genetic exchange between thermophilic anaerobic bacteria and yeasts.