An ABC Transporter from Bacillus thuringiensis Is Essential for β-Exotoxin I Production

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Abstract
β-Exotoxin I is a nonspecific insecticidal metabolite secreted by some Bacillus thuringiensis strains. Several studies of B. thuringiensis strains that have lost the capacity to produce β-exotoxin I have suggested that there is a strong correlation between high levels of β-exotoxin I production and the ability to synthesize crystal proteins. In this study, we showed that a mutant strain, B. thuringiensis 407-1(Cry−)(Pig+), with no crystal gene, produced considerable amounts of β-exotoxin I together with a soluble brown melanin pigment. Therefore, β-exotoxin I production can take place after a strain has lost the plasmids bearing the cry genes, which suggests that these curable plasmids probably contain determinants involved in the regulation of β-exotoxin I production. Using a mini-Tn10 transposon, we constructed a library of strain 407-1(Cry−)(Pig+) mutants. We screened for nonpigmented mutants with impaired β-exotoxin I production and identified a genetic locus harboring two genes (berA and berB) essential for β-exotoxin I production. The deduced amino acid sequence of the berA gene displayed significant similarity to the ATP-binding domains of the DRI (drug resistance and immunity) family of ATP-binding cassette (ABC) proteins involved in drug resistance and immunity to bacteriocins and lantibiotics. The berB gene encodes a protein with six putative transmembrane helices, which probably constitutes the integral membrane component of the transporter. The demonstration that berAB is required for β-exotoxin I production and/or resistance in B. thuringiensis adds an adenine nucleotide analog to the wide range of substrates of the superfamily of ABC proteins. We suggest that berAB confers β-exotoxin I immunity in B. thuringiensis, through active efflux of the molecule.