Influence of the transposition of the thermostabilizing domain of Clostridium thermocellum xylanase (XynX) on xylan binding and thermostabilization

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.