Conjugative transfer of chromosomal DNA in Streptomyces: molecular principle and its relevance for the evolution of genomes
Final Report Abstract
Horizontal gene transfer is the driving force in the evolution of genomes. This is of particular importance in the mycelial growing Gram-positive soil bacterium Streptomyces. Streptomycetes are the most important producers of antibiotics and other secondary metabolites, whereat each isolate contains in general more than 10 biosynthetic gene cluster (BGC). Streptomycetes developed a unique way of conjugative plasmid transfer, which depends on a single plasmid-encoded protein, the DNA-translocase TraB, which transfers a double-stranded plasmid molecule with nearly 100 % efficiency. By genetic mating experiments with specifically marked S. coelicolor superhost strains we were able to show that nearly each plasmid transfer event is associated with the simultaneous transfer of chromosomal DNA. Beside the plasmid, megabase-sized chromosomal DNA fragments, encoding several BGC, or even the complete chromosome were transferred. The frequency of the DNA-transfer was so high that it could be visualized by fluorescent repressor operator system (FROS) microscope imaging without any prior selection processes. In these matings, the surprisingly high chromosome mobilization frequency of nearly 100 % was caused by the chromosomally encoded BGC of the donor, which exert a positive selection pressure on the BGC transfer into the superhost recipient, which carries deletions of all BGC. When antibiotic production of the donor was prevented or when recipients, which produced the same antibiotics as the donor were used in the mating experiment, the chromosome mobilization rate was reduced to only 0.2 – 0.8 %, since there is no selection pressure under these mating conditions. Apparently, the BGCs of the donor act as non-conventional toxin - antitoxin systems with the produced antibiotic representing the toxin component and the resistance determinant within the BGC acting as the antitoxin. Thus, the BGC of the donor exert a selection pressure for the preferential survival of those recipients that obtained chromosomal DNA fragments containing the respective BGC with its resistance determinant. Since in matings each BGC exerts a selection pressure for its transfer, multiple BGC will accumulate in the recipient genome. The finding that the Streptomyces-specific TraB-plasmid-transfer system is able to mobilize with high frequency huge chromosomal DNA fragments, or even the complete chromosome, and the finding that BGC act as non-conventional toxin-antitoxin systems, that exert a positive selection pressure on their conjugative transfer explains the abundance of BGC in Streptomyces genomes and changes our understanding of the evolution of Streptomyces genomes.
