These observations are consistent with the results of Yamada et al. (14). In addition to tnr, the three loci, TmSSU1, TmFKBP12 and TmKu80, were disrupted (transformation and HI frequencies are shown in Table
2). TmSSU1 is an ortholog of TruSSU1 (from Trichophyton rubrum)/AbeSSU1 (from Arthroderma benhamiae) (34), which encodes a putative sulphite efflux pump. FKBP12 (12-kDa FL506-binding protein) is a peptidyl-prolyl isomerase, a highly conserved protein in mammals and fungi (35). It binds to rapamycin, an antibiotic produced by Streptomyces hygroscopicus (36), and forms complexes that inhibit signal transduction by TOR kinases (37). Ku80, in cooperation with Ku70, encodes key components of the NHEJ pathway involved OSI-906 cell line in DSBR. The TmKu80-knockout mutant showed enhanced homologous recombination GSI-IX purchase frequency (14). All Southern blotting profiles indicated a single copy of homologous integration except for the TmSSU1Δ mutants
produced by TmL28. Five of these latter putative mutants showed an additional ectopic band (data not shown). Moreover, growth restriction of T. mentagrophytes strains was tested on SDA media supplemented with serial concentrations of rapamycin. FKBP12-deficient mutants are viable and resistant to blockage of growth by rapamycin (37). Phenotypic characterization revealed that Interleukin-3 receptor TIMM2789 and TmL28 had hypersensitivity toward rapamycin, even at the lowest concentration used (50 ng/mL rapamycin) (data not
shown). Similarly to the TmFKBP12Δ mutant produced by disruption of TmKu80 (unpublished data), TmF11 and TmLF1 (TmFKBP12-disruptants) were resistant to rapamycin and showed normal growth (data not shown). In a previous study, we demonstrated enhanced gene targeting efficiency in the T. mentagrophytes TmKu80Δ mutant, which is defective in the end-joining pathway (14). We showed that HR occurred at a frequency of only 73%. However, the need for exogenous DNA to integrate at a more efficient HI rate is preferential. In addition, deletion of the KU70:KU80 heterodimer leads to a potential pleiotrophic effect on telomere length homeostasis (38). This prompted us to consider other factors that might control NHEJ in the dermatophyte T. mentagrophytes. The DNA repair mechanism is highly conserved in all organisms. The first step in nonhomologous recombination repair of double strand breaks is binding of KU70-KU80 heterodimers to the broken DNA ends followed by Lig4-Xrcc4 complex joining by BRAC1 domains (4, 39). Thus, DNA ligase IV is involved in the final step of NHEJ. Given the crucial role of Lig4 and the predominance of the NHEJ pathway in filamentous fungi, it is important to determine the HI frequency of exogenous DNA in TMLIG4-deficient mutants.