The constructed large recombinant plasmid could not be transformed into the wild-type Bacillus subtilis N4 by traditional spizizen method. In this study, we optimized the spizizen method by adding organic solvents and lysozyme in preparation and transformation of competent cells. The types and concentrations of organic solvent, the concentrations of lysozyme, co- culture time of plasmids and competent cells were studied for improving transformation efficiency to construct Bacillus subtilis N4 PHT01-nit. Aft...