To engineer endophytic Enterobacter cloacae as a biocontrol agent against banana fusarium wilt, a promoter-probe plasmid pUCK was constructed to identify a strong promoter to express disease resistance genes. Using a kanamycin resistance gene for selection, 10 fragments with strong promoter activity were identified from the genome of the E. cloacae KKWB-10 strain. The regions of these 10 fragments that were the primary contributors to the promoter function were identified, and their promoter activities were further evaluated using green fluorescent protein (GFP) as a reporter gene. Fragment 132aaEuro(3) drove the highest level of GFP activity when the bacteria bearing the fragments were cultured in Luria-Bertani and banana stem extract media. The GFP-expressing strain harboring fragment 132aaEuro(3) (K-pUCK7-132aaEuro(3)-GT) was then inoculated into banana plantlets (about 1 x 10(7) CFU per plant) to verify the activity of fragment 132aaEuro(3) in planta. Ten days after inoculation, tissue sections of these banana plantlets were observed by laser confocal scanning microscope. Green fluorescence was observed in the tissues of banana plantlets inoculated with K-pUCK7-132aaEuro(3)-GT but not in uninoculated controls. These results suggest that fragment 132aaEuro(3) possesses strong promoter activity when its host strain colonizes the banana plants and can be used to engineer endophytic E. cloacae KKWB-10 for biocontrol.