Classical High Frequency of Recombination
strains (HFR) carry the conjugative plasmid at a specific location in the chromosome (Thomas & Nielsen, 2005). Plasmid integration normally occurred via homologous recombination between IS elements. Initiation of rolling-circle replication at the plasmid oriT by the conjugative relaxase creates a linear single-stranded DNA molecule that contains plasmid sequences followed by the chromosomal loci next to the integration site. This strand is guided by the covalently bound relaxase to the recipient, where it can recombine with the chromosome (de la Cruz et al., 2010). Because the Streptomyces DNA-translocase TraB does not have a relaxase activity and most probably does not process the DNA (Reuther et al., 2006a) and because clt is dispensable for the transfer of chromosomal markers (Pettis & Cohen, 1994), the chromosome mobilization mechanism in Streptomyces selleck screening library must be different (Fig. 2). An explanation provides the finding that TraB recognizes 8-bp TRS motifs and that clt-like sequences containing repeated TRS are frequently found in Streptomyces chromosomes (Vogelmann et al., 2011a). Analysis of the Streptomyces coelicolor genomic sequence for pSVH1 clt-like sequences (four copies of the TRS GACCCGGA with a spacing of up to 13 bp, allowing one mismatch) identified 25 hits. These sequences are not part of integrated plasmids
or represent remnants of plasmids, but are often located Gefitinib nmr within genes without disrupting their coding region. These insertions are only found in the respective S. coelicolor genes but not in the corresponding homologues of Streptomyces avermitilis or those of other Streptomyces species, which carry clt-like sequences on other locations (Sepulveda et al., 2011). This demonstrates that these insertions have been acquired later and are probably not involved in the respective enzymatic activities. It is unclear how these insertions have been generated. But with respect to the prevalence of plasmids in Streptomyces, one can speculate that there is an adaptive selection for clt-like
sequences in Streptomyces genomes to benefit from the presence of conjugative plasmids. Pettis & Cohen (1994) clearly demonstrated that TraB is the GPX6 only plasmid-encoded protein required for conjugative transfer of pIJ101. Similarity of TraB to the chromosome segregator proteins FtsK or SpoIIIE suggests a conjugative DNA translocation mechanism for the transfer between a donor and a recipient mycelium that resembles the intracellular segregation of chromosomal DNA during cell division and sporulation. TraB hexamers probably assemble at the plasmid localized clt or, with lower efficiency, at chromosomal clt-like sequences. These hexamers form pore structures in the membrane, which act as molecular motors, energized by ATP hydrolysis and translocate double-stranded DNA to the recipient (Fig. 3). However, this simplified model has drawbacks and leaves several open questions.