Bacillus amyloliquefaciens strains FZBREP and FZBSPA were derived from the wild-type FZB42 by replacement of the native bacilysin operon promoter with constitutive promoters P (repB) and P (spac) from plasmids pMK3 and pLOSS, respectively. These strains contained two antibiotic resistance genes, and markerless strains were constructed by deleting the chloramphenicol resistance cassette and promoter region bordered by two lox sites (lox71 and lox66) using Cre recombinase expressed from the temperature-sensitive vector pLOSS-cre. The vector-encoded spectinomycin resistance gene was removed by high temperature (50 A degrees C) treatment. RT-PCR and qRT-PCR results indicated that P (repB) and especially P (spac) significantly increased expression of the bac operon, and FZBREP and FZBSPA strains produced up to 170.4 and 315.6 % more bacilysin than wild type, respectively. Bacilysin overproduction was accompanied by enhancement of the antagonistic activities against Staphylococcus aureus (an indicator of bacilysin) and Clavibacter michiganense subsp. sepedonicum (the causative agent of potato ring rot). Both the size and degree of ring rot-associated necrotic tubers were decreased compared with the wild-type strain, which confirmed the protective effects and biocontrol potential of these genetically engineered strains.