Influence of the Transposition of the Thermostabilizing Domain of Clostridium thermocellum Xylanase (XynX) on Xylan Binding and Thermostabilization

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Abstract
A xylanase gene, xynX , of Clostridium thermocellum had one thermostabilizing domain (TSD) between the signal peptide sequence and the catalytic domain (CD). The TSD of a truncated xylanase gene, xynX ′ TSD-CD , was transpositioned from the N terminus to the C terminus of the CD by overlapping PCRs, and a modified product, xynX ′ CD-TSD , was constructed. XynX′ TSD-CD had a higher optimum temperature (70°C versus 65°C) and was more thermostable (residual activity of 68% versus 46% after a 20-min preincubation at 70°C) than the one without the TSD, XynX′ CD . However, the domain-transpositioned enzyme, XynX′ CD-TSD , showed a lower optimum temperature (30°C) and thermostability (20%) than XynX′ CD . Both XynX′ TSD-CD and XynX′ CD-TSD showed significantly higher binding capacity toward xylan than XynX′ CD , and the domain transposition did not cause any change in the binding ability. XynX′ TSD-CD and XynX′ CD-TSD also showed considerable binding to lichenan but not to carboxymethyl cellulose and laminarin. XynX′ TSD-CD and XynX′ CD-TSD had higher activities for insoluble xylan than XynX′ CD , while XynX′ CD was more active against soluble xylan than XynX′ TSD-CD and XynX′ CD-TSD . These results indicate that the TSD of XynX has dual functions, xylan binding and thermostabilization, and the domain should also be classified as a xylan-binding domain (XBD). The binding capacity of the XBD was not affected by domain transpositioning within the gene.