The ins and outs of DNA transfer in bacteria - PubMed (original) (raw)

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The ins and outs of DNA transfer in bacteria

Inês Chen et al. Science. 2005.

Abstract

Transformation and conjugation permit the passage of DNA through the bacterial membranes and represent dominant modes for the transfer of genetic information between bacterial cells or between bacterial and eukaryotic cells. As such, they are responsible for the spread of fitness-enhancing traits, including antibiotic resistance. Both processes usually involve the recognition of double-stranded DNA, followed by the transfer of single strands. Elaborate molecular machines are responsible for negotiating the passage of macromolecular DNA through the layers of the cell surface. All or nearly all the machine components involved in transformation and conjugation have been identified, and here we present models for their roles in DNA transport.

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Figures

Fig. 1

Fig. 1

Comparison of DNA processing and transfer during transformation and conjugation. (A) In transformation, dsDNA substrates are converted to single-stranded transfer intermediates for transport across the cytoplasmic membrane. (B) For conjugation, surface adhesins or conjugative pili mediate donor-target cell contacts. Initial reactions involve the formation of a relaxase–T-DNA transfer intermediate (green dot joined to black line) and tight mating junctions. Substrate transfer is probably mechanistically conserved in bacteria, although Gram-negative systems can deliver substrates, including proteins (green dots), to phylogenetically diverse target cells (–80).

Fig. 2

Fig. 2

DNA uptake during transformation in B. subtilis. The uptake machinery is preferentially located at the cell poles. The Ψ-prepilins are processed by the peptidase and translocate to the outer face of the membrane. With the aid of the other ComG proteins, the major Ψ-pilin ComGC assembles into the Ψ-pilus, which attaches exogenous DNA via a hypothetical DNA binding protein. Retraction of the Ψ-pilus, driven by the proton motive force, and DNA binding to the receptor (ComEA) are required to transport one strand of DNA through the membrane channel (ComEC) while the other is degraded by an unidentified nuclease. The helicase/DNA translocase (ComFA) assists the process, along with ssDNA binding proteins that interact with the incoming DNA. RecA forms a filament around the ssDNA, and mediates a search for homology with chromosomal DNA. ADP, adenosine diphosphate; Pi, inorganic phosphate; PMF, proton motive force; ssb, single-stranded DNA binding protein.

Fig. 3

Fig. 3

Conjugative DNA transfer through the A. tumefaciens VirB/D4 system. DNA and protein substrates dock initially at the VirD4 receptor, then transfer in succession to the channel components VirB11 ATPase, VirB6, and VirB8, and finally VirB2 and VirB9. Three ATPases (VirD4, VirB4, VirB11) energize DNA substrate transfer through the membrane translocase comprised of either or both VirD4 and VirB6. The DNA substrate translocates to the cell surface via a channel comprised of VirB2 pilin and secretin-like VirB9. ATP energy also induces a structural transition (double-ended arrow) in VirB10 to mediate substrate transfer to the distal portion of the secretion channel.

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