PubMed · 1064555
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D R Helinski. 1976. Plasmids: introductory remarks.. https://pubmed.ncbi.nlm.nih.gov/1064555/
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Integrative and conjugative elements (ICEs) are typically found integrated in a bacterial host chromosome. They can excise, replicate, and transfer from cell to cell. Many contain genes that confer phenotypes to host cells, including antibiotic resistances, specialized metabolisms, phage defense, and symbiosis or pathogenesis determinants. Recent studies revealed that at least three ICEs (ICEclc, Tn916, and TnSmu1) cause growth arrest or death of host cells upon element activation. This review highlights the complex interactions between ICEs and their hosts, including the recent examples of the significant costs to host cells. We contrast two examples of killing, ICEclc and Tn916, in which killing, respectively, benefits or impairs conjugation and emphasize the importance of understanding the impacts of ICE-host relationships on conjugation. ICEs are typically only active in a small fraction of cells in a population, and we discuss how phenotypes normally occurring in a small subset of host cells can be uncovered.
Conjugative coupling proteins (CPs) are proposed to play a role in connecting the relaxosome to a type IV secretion system (T4SS) during bacterial conjugation. Here we present biochemical and genetic evidence indicating that the prototype CP, TrwB, interacts with both relaxosome and type IV secretion components of plasmid R388. The cytoplasmic domain of TrwB immobilized in an affinity resin retained TrwC and TrwA proteins, the components of R388 relaxosome. By using the bacterial two-hybrid system, a strong interaction was detected between TrwB and TrwE, a core component of the conjugative T4SS. This interaction was lost when the transmembrane domains of either TrwB or TrwE were deleted, thus suggesting that it takes place within the membrane or periplasmic portions of both proteins. We have also analyzed the interactions with components of the related IncN plasmid pKM101. Its CP, TraJ, did not interact with TrwA, suggesting a highly specific interaction with the relaxosome. On the other side, CPs from three different conjugation systems were shown to interact with both their cognate TrwE-like component and the heterologous ones, suggesting that this interaction is less specific. Mating experiments among the three systems confirmed that relaxosome components need their cognate CP for transfer, whereas T4SSs are interchangeable. As a general rule, there is a correlation between the strength of the interaction seen by two-hybrid analysis and the efficiency of transfer.
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