Cloning, characterization and phylogenetic relationships of cel5B, a new endoglucanase encoding gene from Thermobifida fusca

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Abstract
Thermobifida fusca, a thermophilic, aerobic, cellulolytic bacterium has a highly complex cellulase system comprising three endoglucanases, two exoglucanases and one processive endoglucanase. Zymogram analysis indicated that additional cellulases may exist in T. fusca strain TM51, therefore a TM51 expression library was prepared in Streptomyces lividans TK24 and screened for hydrolases. A new endoglucanase gene, named Tf cel5B, was identified. Heterologous Cel5B, produced in S. lividans, had temperature and pH optima of 77 °C and 8.2, respectively and retained more than 60% of its activity after 24 h incubation at 60 °C. Domain analysis revealed an N‐terminal catalytic domain with homology to known endoglucanases in family GH5 and a C‐terminal cellulose binding module III domain (CBD). Comparing the domain structures of all seven known T. fusca cellulases showed, that the cellulase system of this organism consists of pairs of enzymes from the same GH family, including Cel5A – Cel5B, Cel6A – Cel6B and Cel9A – Cel9B plus a single family GH48 enzyme (Cel48A). Furthermore, the catalytic and substrate binding domains of enzymes, belonging to the same GH family were arranged in opposite orientations. Phylogenetic comparisons of the catalytic domain sequences of the T. fusca cellulases to other family GH5, GH6, GH9 and GH48 cellulases of bacterial origin revealed that the enzyme pairs in the same GH family are not closely related to each other, instead they showed significant similarities to various cellulase enzymes from taxonomically distinct organisms. Therefore, the complex and highly efficient cellulase system of T. fusca seems to be evolved as a result of horizontal gene transfers rather than gene duplication events.

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