Conjugative transfer of chromosomal DNA in Streptomyces: molecular principle and its relevance for the evolution of genomes

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Conjugation, the transfer of DNA between bacteria by direct cell-to-cell contact is a major route of horizontal gene transfer (HGT). Antibiotic producing soil bacteria of the genus Streptomyces developed a unique conjugation system, transferring double-stranded plasmid molecules with nearly 100 % efficiency. This transfer is directed by a single plasmid-encoded protein, the DNA-translocase TraB, which binds to a specific non-coding plasmid region (clt). Plasmid transfer is associated with the simultaneous transfer of chromosomal DNA. This Chromosome Mobilizing Ability (CMA) has been extensively used for decades to establish genetic linkage maps of Streptomyces chromosomes, however its underlying mechanism is completely unknown. We observed the transfer of megabase-sized DNA-fragments and showed that the Streptomyces chromosome exploits the plasmid-encoded transfer apparatus, thereby acting itself as a mobile genetic element. We aim to elucidate the molecular principles of CMA by identifying and characterizing chromosomal clt-like binding sites of TraB and by revealing other cellular functions involved in CMA by insertion tracking (Tn-seq). We want to determine the size and the extent of the transferred chromosomal DNA by whole genome sequencing of transconjugants and study the relevance of CMA for the transfer and dissemination of biosynthetic gene clusters and resistance determinants.