Construction of the Harpin expression vector pGAprEHS. The amplified partial FZB42 aprE gene sequence was cloned into pGEMT T/A vector resulting in pGEMT-aprE. Then, the gene encoding HpaGXooC was inserted into the double-digested (HpaI and ClaI) plasmid pGEMT-aprE resulting in pGAprEH. Finally, the antibiotic-resistance marker Spec was amplified from plasmid pIC333 and inserted into vector pGAprEH, yielding integration vector pGAprEHS. In addition, the aprE knockout vector pGAprES was obtained when the antibiotic cassette was inserted into plasmid pGEMT-aprE.