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Category: Clinical Microbiology
Conjugation in Gram-Positive Bacteria, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818982/9781555818975_Chap14-1.gif /docserver/preview/fulltext/10.1128/9781555818982/9781555818975_Chap14-2.gifAbstract:
Conjugative transfer is an important driver in evolution, enabling bacteria to acquire new traits ( 1 , 2 , 3 , 4 , 5 ). During conjugative transfer, DNA translocation across the cell envelopes of two cells forming a mating pair is mediated by two types of mobile genetic elements: conjugative plasmids and integrating conjugative elements (ICEs) ( 1 , 5 , 6 , 7 , 8 ). Most conjugative plasmids apply a sophisticated multiprotein secretion apparatus, the so-called type IV secretion system (T4SS) to transfer DNA to a recipient cell ( 9 , 10 , 11 , 12 , 13 ). Conjugative T4SSs of Gram-positive (G+) bacteria exhibit considerable similarities to their Gram-negative (G–) counterparts; the first steps processing the plasmid DNA to be transferred with the relaxase, covalently attached to its 5′ end, are virtually identical ( 11 , 14 , 15 , 16 ). However, the actual DNA translocation process including the passage of the cell envelope of the donor and the recipient cell appears to differ considerably between G+ and G– bacteria. This might be due to the differences in the structure of the cell envelope: cytoplasmic membrane followed by a thick multilayered peptidoglycan (PG) in G+ bacteria versus a two-membrane configuration with periplasmic space and thin PG layer between the two membranes in G– bacteria. Therefore, it is not surprising that homologs of VirB7, VirB9, and VirB10 proteins identified as actual G– T4SS channel components ( 17 , 18 , 19 , 20 , 21 , 22 , 23 ) have not been detected so far in G+ T4SSs.
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Genetic organization of the pIP501 tra operon. Proteins with sequence similarities with the corresponding A. tumefaciens Ti-plasmid VirB/D4, E. faecalis pCF10, and C. perfringens pCW3 T4SS proteins are in blue; the potential two-protein-coupling protein (consisting of TraIpIP501 and TraJpIP501) is indicated with brackets; relations based on structure (TraM C-terminal domain, VirB8-like [ 54 ]) and domain prediction (TraL, VirB6-like) based similarities are in yellow; the gene encoding the putative relaxase is in green. The respective protein families are indicated. Ptra, tra operon promoter. The genes of the pIP501 tra region are drawn to scale. Put., putative.
Model of the pIP501 DNA transfer pathway. First, oriT pIP501 is bound by the relaxase TraA. After being nicked, the single-stranded plasmid is recruited to the putative transfer channel (modified from reference 17) via the putative two-protein coupling protein TraJ. Decreased shading of PG symbolizes TraG-mediated local opening of PG. The localization and orientation of the T4SS proteins is based on in silico predictions and localization studies ( 52 , 54 ). The N terminus of the T4SS proteins is marked (N). Arrows indicate protein-protein interactions determined by yeast-two-hybrid studies and validated by pull-down assays ( 27 ), as well as interactions found by using the Thermofluor method (Goessweiner-Mohr et al., unpublished data). The thickness of the arrows marks the strength of the detected interactions. The putative function of key members of the pIP501 tra operon in the DNA secretion process is indicated. PG, peptidoglycan; CM, cytoplasmic membrane; CP, cytoplasm.
Comparison of the domain arrangement and classification of putative lytic transglycosylases from G+ and G– putative T4SSs. TraG (pIP501), TcpG (pCW3), VirB1 (Ti-plasmid), and PrgK (pCF10) were chosen as representatives of their respective classes. The potential SLT and CHAP(-like) domains were assigned according to secondary structure predictions with PSIPred ( 131 ). TMHs were annotated with HMMTOP ( 53 ).
Secondary structure-based classification of lytic transglycosylases. A, class α, B, class β, C, class γ, D, class δ lytic transglycosylases. Secondary structure (PSIpred) and TM motif (HMMTOP) prediction for G– and G+ lytic transglycosylases from conjugative plasmids, transposons, ICEs, and GIs; alpha helices (blue), beta strands (red), and TM motifs (boxes) are highlighted; the putative N-terminal ends of the SLT and CHAP(-like) domains are indicated.
Secondary structure-based classification of lytic transglycosylases. A, class α, B, class β, C, class γ, D, class δ lytic transglycosylases. Secondary structure (PSIpred) and TM motif (HMMTOP) prediction for G– and G+ lytic transglycosylases from conjugative plasmids, transposons, ICEs, and GIs; alpha helices (blue), beta strands (red), and TM motifs (boxes) are highlighted; the putative N-terminal ends of the SLT and CHAP(-like) domains are indicated.
Secondary structure-based classification of lytic transglycosylases. A, class α, B, class β, C, class γ, D, class δ lytic transglycosylases. Secondary structure (PSIpred) and TM motif (HMMTOP) prediction for G– and G+ lytic transglycosylases from conjugative plasmids, transposons, ICEs, and GIs; alpha helices (blue), beta strands (red), and TM motifs (boxes) are highlighted; the putative N-terminal ends of the SLT and CHAP(-like) domains are indicated.
Secondary structure-based classification of lytic transglycosylases. A, class α, B, class β, C, class γ, D, class δ lytic transglycosylases. Secondary structure (PSIpred) and TM motif (HMMTOP) prediction for G– and G+ lytic transglycosylases from conjugative plasmids, transposons, ICEs, and GIs; alpha helices (blue), beta strands (red), and TM motifs (boxes) are highlighted; the putative N-terminal ends of the SLT and CHAP(-like) domains are indicated.
Comparison of the domain arrangement and classification of VirB6-like proteins from G+ and G– putative T4SSs. TraL (pIP501), PrgH (pCF10), TcpH (pCW3), VirB6 (Ti-plasmid), TraD (pKM101), TraG (R1 plasmid), and TrbP (pBP136) were selected as representatives of their respective classes. Predicted TMHs (HMMTOP [ 53 ]) are represented as gray boxes.
Classification of TraG-like and other lytic transglycosylases