Natural transformation in Helicobacter pylori: DNA transport in an unexpected way

Abstract

Response from Hofreuter and HaasWe agree with the model for natural transformation competence in Helicobacter pylori put forward by Smeets and Kusters. It summarizes what we know about the components comprising type IV transport systems and how we assume that DNA might enter the H. pylori cytoplasm. However, although it serves as a working model, we must bear in mind that for ComB, or indeed for any other DNA-transport system, there is no experimental evidence to prove that DNA moves through the postulated transmembrane pore. What is the novel aspect in the H. pylori transformation system?Until recently, the mechanisms of DNA transfer involved in conjugation and transformation were presumed to be unrelated, although it was known that a competence protein in Bacillus subtilis has homology to TrwD, an ATPase essential for conjugation of plasmid R388 [1xDNA uptake in bacteria. Dubnau, D. Annu. Rev. Microbiol. 1999; 53: 217–244Crossref | PubMed | Scopus (405)See all References][1]. The H. pylori transformation system now fills the gap between these two DNA-transfer systems, as it is the first system for uptake of exogenous DNA that does not consist of structural components homologous to type IV pili but to type IV secretion systems, as Smeets and Kusters discuss.The novel transformation apparatus of H. pylori is assembled by ComB proteins with homology to VirB4, -7, -8, -9 and -10 of the Agrobacterium tumefaciens virB operon. For T-DNA transfer into plant cells, VirB2–VirB11 and VirD4 are essential. Interestingly, a subset of components of the T-DNA translocon, including the membrane-spanning channel-forming core components, VirB7–VirB10, seems to be involved in plasmid DNA uptake in the recipient cell during conjugation [2xThe Ti plasmid increases the efficiency of Agrobacterium tumefaciens as a recipient in virB-mediated conjugal transfer of an IncQ plasmid. Bohne, J. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 7057–7062Crossref | PubMed | Scopus (45)See all References][2]. This situation is reminiscent of the uptake of DNA by natural transformation in H. pylori. In the first ‘mobile’ type IV-related transformation system, located on a plasmid in Campylobacter jejuni [3xInvolvement of a plasmid in virulence of Campylobacter jejuni 81-176. Bacon, D.J. et al. Infect. Immun. 2000; 68: 4384–4390Crossref | PubMed | Scopus (235)See all References][3], the core components were also found to be essential for transformation, although not a VirB11-homologous ATPase.Now, as it appears that certain transformation systems resemble an inverse conjugation process, several pertinent questions arise for future research. What are the energy requirements for DNA import? Are any of the type IV-related ATPases essential for energizing the system? Are the proton-motive force and ΔpH necessary for DNA import into H. pylori, as reported for B. subtilis [4xRole of proton motive force in genetic transformation of Bacillus subtilis. van Nieuwenhoven, M.H. et al. J. Bacteriol. 1982; 151: 771–776PubMedSee all References][4]? Is DNA import in H. pylori a one-step transfer process from the cell surface into the cytoplasm, as suggested for the A. tumefaciens T-DNA transfer system and plasmid-conjugation systems, or do periplasmic intermediates exist, as described for the translocation of pertussis toxin by the type IV secretion system in Bordetella pertussis [5xBiochemistry of type IV secretion. Burns, D.L. Curr. Opin. Microbiol. 1999; 2: 25–29Crossref | PubMed | Scopus (86)See all References][5]? Is DNA taken up as a single- or double-strand, as naked DNA, or as a DNA–protein complex? Are DNA-stabilising single stranded-DNA-binding proteins and pilot proteins involved, analogous to TraI/VirD2 during conjugation and/or T-DNA transfer?All naturally competent bacteria characterized thus far import single-stranded DNA. As H. pylori strains possess many different restriction–modification systems to degrade double-stranded DNA, it is likely that H. pylori also takes up single-stranded DNA, which might be a better substrate for integration into the chromosome by homologous recombination.