Alternative Ways to Exchange DNA: Unconventional Conjugation Among Bacteria
In addition to asexual reproduction, many prokaryotes contain specific apparatus evolved to exchange DNA through their envelopes by direct cell-to-cell contacts, allowing the horizontal transfer of genes, usually between phylogenetically related species in a sexual-like process. These contact-dependent processes are called “conjugation” to differentiate them from other ways of DNA acquisition like transformation, mediated by DNA import systems, or transduction, mediated by defective phages.
The conjugation apparatus has been studied in great detail in Proteobacteria. As described in previous chapters, it is based on a DNA mobilization apparatus that selects and cuts one of the strands of the transferred DNA in the donor strain at a specific site (oriT), binds a protein covalently to the 5′ extreme, and delivers the ssDNA–protein complex to a type IV secretion system (T4SS), which is responsible for its injection into the recipient cell. However, the absence of homologues to essential components of these elements in the genomes of many bacteria in which conjugation has been described to occur soon revealed the existence of unconventional conjugation mechanisms. Conjugation in Streptomyces involves the transfer of dsDNA instead of ssDNA and depends on a single DNA membrane translocase. In mycobacteria and mycoplasma, conjugation leads to the simultaneous transfer of large DNA fragments from several points in the chromosome and the generation of diverse chimeric progeny. Also in Thermus thermophilus, a transformation-dependent conjugation process allows the simultaneous transfer of every gene in a bidirectional process. In this chapter, we will focus on these so-called “unconventional” pathways of conjugation which, at the end, could be much more frequent as originally thought.
KeywordsConjugation Unconventional Thermus Streptomyces Mycoplasma Mycobacterium
This work was supported by grant BIO2016-77031-R from the Spanish Ministry of Economy and Competitiveness and grant no. 685474 from Horizon 2020 research and innovation program of the European Union. An institutional grant from Fundación Ramón Areces to the CBMSO is also acknowledged.
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