Construction of the Harpin expression vector pUNprEKHS. The resistance marker lox-Km was obtained from plasmid pBT2-arcA (Leibig et al. ) after PstI digestion and subsequent cloning into pUC18. The resulting plasmid was named pUK. The upstream and downstream sequences of the nprE gene were inserted into plasmid pUK, yielding nprE knockout vector pUNprEK. The Harpin gene fused with the P43 promoter and the nprE signal peptide was inserted into the Hinc II site of pUNprEK, yielding pUNprEKHS.