The wild-type xylanase gene xyl7 was cloned from the metagenomic
fosmid library using primers xyl7-1F and xyl7-1R. The signal
peptides and protein family domains were predicted by Signal 4.1
[15] and Pfam [16], respectively. The signal peptide was removed
from the xyl7 gene in all constructs. Four types of fragment that
included the GH11-conserved domain were cloned (Fig. 1): xyl7-1F
and xyl7-2R primers were used to amplify the truncated fragment
XYL7-Tr1 [amino acids (aa) 20–253], xyl-1F and xyl-3R were used
to amplify XYL7-Tr2 (aa 20–286), xyl-1F and xyl-4R were used to
amplify XYL7-Tr3 (aa 20–290) and xyl-2F and xyl-4R were used
to amplify XYL7-Tr4 (aa 33–253) (Table 1). The forward and the
reverse primers contained NdeI and EcoRI restriction sites, respectively.
The PCR products were double digested and ligated into
pET28a, and the resulting plasmids were used to transform E. coli
TOP10 for plasmid amplification.