, 2003). The small size of the plasmid region determining conjugative transfer already indicated that the Streptomyces DNA transfer mechanism must differ considerably from the known conjugation systems of other bacteria, involving a conjugative relaxase and a complex type IV protein secretion system (Chen et al., 2005; de la Cruz et al., 2010). Characterization of several Streptomyces plasmids by subcloning and linker insertions revealed a plasmid region of about 3 kb being essential for transfer, while the adjacent region affected only the size of the pock structures (Kieser et al., 1982; Kataoka et al., 1991; Servin-Gonzalez et al., 1995; Reuther

et al., 2006a). When the nucleotide sequence of the Streptomyces lividans plasmid pIJ101 SP600125 mouse was available (Kendall & Cohen, 1988) learn more as the first complete sequence of a conjugative plasmid from a Gram-positive bacterium, it was realized that korA (traR) encoded a

transcriptional regulator of the GntR family, while a small region of the KilA (TraB) protein showed some similarity to the FtsK protein involved in cell division and chromosome segregation (Begg et al., 1995; Wu et al., 1995; Sherratt et al., 2010). Pettis & Cohen (1994) demonstrated that beside the TraB protein, a small non-coding plasmid region of about 50 bp was required for the transfer of plasmid pIJ101, the cis-acting-locus of transfer (clt). When clt was inserted into a nontransferable plasmid, this plasmid could be mobilized,

if TraB was provided in trans. Interestingly, clt was only required for plasmid transfer but was dispensable for the mobilization of chromosomal markers (Pettis & Cohen, 1994), indicating that clt does not represent a classical origin of transfer (oriT). The clt regions of different Streptomyces plasmids do not show any sequence similarity, but often contain repetitive sequences that have CYTH4 the ability to form secondary structures (Franco et al., 2003; Vogelmann et al., 2011a). The first experimental evidence on the novel mechanism of the Streptomyces conjugative DNA transfer system came from the work of Possoz et al. (2001) by demonstrating that conjugative transfer of the Streptomyces ambofaciens plasmid pSAM2 was sensitive to the presence of the SalI restriction/modification system in the recipient. In this study, a pSAM2 derivative could not be transferred into S. lividans TK23 expressing SalI, whereas pSAM2 was efficiently transferred to TK23 lacking the SalI restriction system. Because the transferred DNA was obviously degraded by SalI and because SalI recognizes only double-stranded DNA as substrate but not single-stranded DNA, the incoming DNA must be double-stranded.