TraB is a hexameric pore-forming ATPase that resembles the chromosome segregator protein FtsK ABT-199 order and translocates DNA by recognizing specific 8-bp repeats present in the plasmid clt locus. Mobilization of chromosomal genes does not require integration of the plasmid, because TraB also recognizes clt-like sequences distributed all over the chromosome. Mycelium-forming actinomycetes

do not divide by binary fission but grow by apical tip extension and undergo a complex life cycle ending in sporulation (Flardh & Buttner, 2009). They are well known for the production of antibiotics, a feature probably developed to inhibit competitors in the soil community (Allen et al., 2010). During evolution of the antibiotic biosynthetic gene clusters, they also evolved specific resistance

genes as a part of the cluster to protect themselves from their own compounds. Because a typical Streptomyces strain contains 10–20 different gene clusters for the production of antibiotics and other bioactive secondary metabolites (Bentley et al., 2002; Medema et al., 2011), streptomycetes form a huge reservoir of antibiotic resistance genes in the soil, which can be passed to other bacteria by horizontal gene transfer (D’Costa et al., 2006; Allen et al., 2010). Therefore, the antibiotic KU-57788 ic50 producers not only compete with other organisms by the production of antimicrobial compounds but they also provide resistance genes that can help others to survive. In Streptomyces and related actinomycetes, even small multi-copy plasmids of < 10 kb in size are normally self-transmissible and able to mobilize chromosomal resistance genes and auxotrophic markers (Kieser et al., 1982; Kataoka et al., 1991; Servin-Gonzalez et al., 1995). These plasmids are normally cryptic and do not confer phenotypic traits (Hopwood

& Kieser, 1993; Vogelmann et al., 2011a). Efficiency of transfer reaches nearly 100% and between 0.1% and 1% of MG-132 solubility dmso the transconjugants obtain chromosomal fragments during mating (Kieser et al., 1982). DNA transfer takes place only on solid surfaces in the early growth phase of the life cycle, when Streptomyces grows as substrate mycelium (Pettis & Cohen, 1996; Possoz et al., 2001). The transfer determinants of many Streptomyces plasmids were initially identified as killing functions (kilA, traB), which could only be subcloned in the presence of the corresponding killing override (korA, traR) region (Kendall & Cohen, 1987; Hagege et al., 1993; Reuther et al., 2006a). Probably due to the toxic effects of the transfer determinants, plasmid transfer is associated with the formation of so-called pock structures having a diameter of 1–3 mm. Pocks are formed when donor spores germinate on a lawn of a plasmid-free recipient. Pocks represent temporally retarded growth inhibition zones and indicate the area, where the recipient mycelium has obtained a plasmid by conjugation (Fig. 1).