Dilution aliquots were plated in parallel on LB agar and LB agar containing 5 μM CmC. From the 5 μM CmC plate, 24 clones were isolated and found to be tryptophane auxotroph, proving by this genetic marker their descent from strain 168. Two independently isolated resistant clones designated 8R and IR revealed no cross-resistance
against most antibiotics tested, either structurally related or different (summarized in Supporting Selleck Veliparib Information, Table S2). However, they were more resistant, in particular, against doxorubicin, a hydrophobic polyketide cancer antibiotic (Table S2). Both mutants grew in the presence of 5 μM CmC, and this resistance was unchanged after propagation for >20 generations in the absence of the drug. In contrast, Ohki & Tatenu (2004) obtained their spontaneous multidrug-resistant mutant on the bmr3 gene in a one-step procedure. The genomes of the resistant clones 8R and IR as well as the parent strain B. subtilis 168 were
sequenced to near completion and compared with the reference GenBank: AL009126.3 database (Srivatsan et al., 2008; Barbe et al., 2009). Two of the mutations could be confirmed by PCR amplification and sequencing. Sequence comparison localized these two mutations 40 bp apart in the 5′ noncoding region of the yvcC=bmrA gene (Fig. 1). In S. tendae, the producer of the cervimycin complex, self-resistance is facilitated by a member of the MFS superfamily,
possibly extruding cervimycin (M. Unger & C. Hertweck, unpublished data). BmrA (Steinfels et al., 2002; Orelle et al., 2003) belongs to the largest gene class of see more ATP-dependent ABC exporters in B. subtilis and was found to be expressed constitutively (Steinfels et al., 2004). Previously, the in vitro transport of Hoechst 33342, doxorubicin, 7-aminoactinomycin and EtBr by Dichloromethane dehalogenase BmrA was shown using membrane vesicles, whereas reserpine inhibited the EtBr-efflux (Steinfels et al., 2004). The bmrA knockout mutant (Steinfels et al., 2002) had a twofold reduced resistance to CmC compared with B. subtilis 168. This supports the conclusion that this gene is responsible for CmC resistance, but also indicates the involvement of more factors contributing to the basal resistance. A PCR fragment of bmrA obtained from mutant 8R genomic DNA with primers px yvcC-F/-R1 transformed B. subtilis 168 to 5 μg mL−1 CmC resistance. In accordance with published data (Steinfels et al., 2004), we found additionally that in the presence of 50 mg L−1 reserpine, the mutant was unable to grow in the presence of CmC. From these data, we could conclude that the ABC transporter BmrA is responsible for the CmC resistance of the mutants. So far, reports on spontaneous constitutively resistant mutants in Gram-positive bacteria revealing overexpression due to promoter mutations are rare (Piddock, 2006). One case was reported after growing B.