These studies identified the very C-terminal end of TraB forming a wHTH fold as being responsible for clt recognition. Further studies even narrowed down the TRS recognition region to helix α3 of the wHTH fold. Exchange of only 13 aa of TraBpSVH1 against the 13 aa corresponding to helix α3 of TraBpIJ101 switched clt recognition. The chimeric protein was no longer able to bind to the clt of pSVH1 but shifted the clt fragment of pIJ101 (Vogelmann et al., 2011a). Generation of pock structures during Streptomyces conjugation has been interpreted as the result of MAPK inhibitor intramycelial plasmid

spreading following the primary DNA transfer from a donor into the recipient (Hopwood & Kieser, 1993; Grohmann et al., 2003). Whereas plasmid transfer from a donor into the recipient requires only TraB, plasmid spreading involves five to seven plasmid-encoded proteins (Spd) in addition

to TraB. This probably reflects the challenge to cross the septal cross-walls. The Spd proteins have no significant similarity to any functionally characterized protein complicating prediction AZD2281 order of their putative function. Inactivation of a single spd gene reduces the size of the pock structures (Kieser et al., 1982; Kataoka et al., 1994; Servin-Gonzalez et al., 1995; Reuther et al., 2006a). Only few reports address the biochemical characterization of the Spd proteins and their molecular function is more or less unknown. Genetic organization of the spd genes with overlapping stop and start codons, analysis of protein–protein interaction by chemical crosslinking,

bacterial two-hybrid analysis or copurification experiments indicated that the before Spd proteins form a multiprotein complex with TraB (Tiffert et al., 2007) (Thoma, Guezguez and Muth, unpublished). Intramycelial plasmid spreading might also contribute to the stable maintenance of Streptomyces plasmids, because hyphal compartments that have lost a plasmid can recover a plasmid from the neighbouring compartment. In agreement with this hypothesis, a clear effect of spd1 inactivation on stable maintenance of the linear plasmid SLP2 was reported (Hsu & Chen, 2010). Streptomyces plasmids contribute to the evolution and shaping of the chromosome in different ways (Medema et al., 2010). Linear plasmids can recombine with the chromosome. Because the Streptomyces chromosome is normally linear (Lin et al., 1993), this results in the exchange of the ends, creating plasmids that carry chromosomal DNA. These plasmids can be transferred by conjugation to new Streptomyces species, where they can replicate either autonomously or recombine again with the chromosome. But also circular plasmids have been reported to mobilize chromosomal fragments with high efficiency (Kieser et al., 1982; Hopwood & Kieser, 1993).