J Bacteriol, June 1998, p. 3091-3099, Vol. 180, No. 12
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Institute of Molecular Genetics, Russian Academy of
Science, 123182 Moscow, Russia,1 and
Institute for Microbiology,
Received 29 December 1997/Accepted 16 April 1998
The nucleotide sequence of the Clostridium thermocellum
F7 cbhA gene, coding for the cellobiohydrolase CbhA, has
been determined. An open reading frame encoding a protein of 1,230 amino acids was identified. Removal of a putative signal peptide yields
a mature protein of 1,203 amino acids with a molecular weight of 135,139. Sequence analysis of CbhA reveals a multidomain structure of
unusual complexity consisting of an N-terminal cellulose binding domain
(CBD) homologous to CBD family IV, an immunoglobulin-like
-barrel
domain, a catalytic domain homologous to cellulase family E1, a
duplicated domain similar to fibronectin type III (Fn3) modules, a CBD
homologous to family III, a highly acidic linker region, and a
C-terminal dockerin domain. The cellulosomal localization of CbhA was
confirmed by Western blot analysis employing polyclonal antibodies
raised against a truncated enzymatically active version of CbhA. CbhA
was identified as cellulosomal subunit S3 by partial amino acid
sequence analysis. Comparison of the multidomain structures indicates
striking similarities between CbhA and a group of cellulases from
actinomycetes. Average linkage cluster analysis suggests a coevolution
of the N-terminal CBD and the catalytic domain and its spread by
horizontal gene transfer among gram-positive cellulolytic bacteria.
*
Corresponding author. Mailing address: Institute for
Microbiology, Technical University Munich, Arcisstrasse 21, D-80290
Munich, Germany. Phone: (089) 2892-2372. Fax: (089) 2892-2360. E-mail: zverlov{at}biol.chemie.tu-muenchen.de.
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