Site-directed mutagenesis of Glu 106 to Phe 106 in the codons of wlac (resulting in “wlacS”), and further deletion mutagenesis of the codons between Leu 351 and Gly 378 of wlacS (resulting in “wlacD”) were also carried out. Sequencing results showed that all mutated genes were found to contain the expected sequences. These genes were then ligated to pTYB12 to create recombinant plasmids pMB171 and pMB172, respectively