Abstract
Bacterial conjugation is an example of macromolecular trafficking between cells and responsible for the spreading of antibiotic resistance among bacteria. It involves translocation of single-stranded DNA across membranes through a type IV secretion system. A coupling protein links the DNA-processing nucleoprotein complex, the relaxosome, with the transport apparatus during cell mating. In Escherichia coli plasmid R388 such a protein is TrwB, a basic integral inner-membrane nucleoside-triphosphate-binding protein. TrwB is the structural prototype for the type IV secretion system coupling proteins, a family of proteins essential for macromolecular transport between cells and export. The structure of a soluble TrwB variant unveils an elongated molecule with six equivalent protein units featuring a spherical quaternary structure, leaving a central channel. The structures of the non-liganded protein and four different complexes with substrate analogues and products allow the precise description of the active site architecture. The active sites are located at the interface between protomers, each of them shaped mainly by residues of one monomer, but including two crucial arginine residues belonging to the adjacent molecule. Upon substrate binding and putative hydrolysis, conformational changes are transferred from the external surface to the interior central channel.
Highlights
Bacterial conjugation is an example of macromolecular trafficking between cells and responsible for the spreading of antibiotic resistance among bacteria
The latter is mediated by conjugative plasmids aimed to transfer DNA unidirectionally between bacteria, from a donor to a recipient cell, which becomes a potential donor
The relaxosome moves to the transport site, and the type IV DNA transport apparatus transfers single-stranded1 DNA or T-strand to the recipient cell
Summary
Bacterial conjugation is an example of macromolecular trafficking between cells and responsible for the spreading of antibiotic resistance among bacteria It involves translocation of single-stranded DNA across membranes through a type IV secretion system. The latter is mediated by conjugative plasmids aimed to transfer DNA unidirectionally between bacteria, from a donor to a recipient cell, which becomes a potential donor It is the main process by which bacterial pathogens acquire antibiotic resistance and constitutes a widespread tool to shuttle genes between different species of bacteria. TrwA is a small-sized tetrameric protein, which binds two sites at oriT and enhances TrwC relaxase activity, while repressing transcription of the trwABC operon [16] These two proteins constitute, together with oriT DNA and the hostencoded integration host factor, the relaxosome of plasmid R388 [12].
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