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Chapter 7 : Conjugation and Genetic Exchange in Enterococci

Authors: Don B. Clewell1, Gary M. Dunny2
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Affiliations: 1: Department of Biologic and Materials Sciences, School of Dentistry, and Department of Microbiology and Immunology, School of Medicine, The University of Michigan, Ann Arbor, MI 48109-1078; 2: Department of Microbiology, University of Minnesota, Minneapolis, MN 55455
Content Type: Monograph
Publication Year: 2002

Category: Bacterial Pathogenesis

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Abstract:

The medical importance of the enterococci is closely related to the propensity of these organisms to participate in the horizontal transfer of determinants for antibiotic resistance and virulence. Conjugative plasmids in enterococci tend to fall into two main groups. Members of one group encode recognition of recipient-produced peptide pheromones that initiate the mating process; the others do not make use of such signals. The pheromone-responding plasmids usually transfer efficiently in broth (liquid) matings, whereas the others transfer relatively poorly under these conditions. An exception is pMGl, a resistance plasmid in that transfers well in broth despite the apparent absence of a pheromone system. Those plasmids that make use of sex pheromones (e.g., pADl and pCFlO) thus far appear to exhibit a narrow host range—primarily and closely related species-although information on this point is very limited. A partial list of pheromone-responding plasmids is presented in this chapter. The formation of mating aggregates relates to the induction of a protein "aggregation substance" (AS), which appears extensively over the donor surface and binds to "enterococcal binding substance" (EBS) on the recipient surface. Conjugative transposons are particularly common in enterococci and streptococci and play an important role in the dissemination of antibiotic resistance in these organisms. It is likely that the enterococci play a significant role as a hardy facultative reservoir of genetic information available to a variety of other genera.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7

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Conjugation and Genetic Exchange in Enterococci
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Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
/content/10.1128/9781555817923.chap7.fig7-1
Figure 1

Expression of pCF10-encoded surface proteins observed by field emission scanning electron microscopy. The cells shown are from cultures of isogenic strains of carrying either the shuttle vector pWM402 (right panel) or pINY1801 (a chimeric plasmid containing the cloned [surface exclusion protein, Sec10]) and [aggregation substance, Asc10] genes of pCFlO cloned into pWM402). The latter plasmid confers constitutive expression of Asc10 and Sec10 on its host cell ( ). The cells were preserved by a cryofixation procedure similar to that described by Erlandsen et al. ( ). Note the smooth appearance of the bacterial cell surface (right panel) and surface display of the plasmid-encoded proteins as discrete entities with well-defined structural features. Micrographs were kindly supplied by Dr. Stanley Erlandsen, University of Minnesota.

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Figure 1

Expression of pCF10-encoded surface proteins observed by field emission scanning electron microscopy. The cells shown are from cultures of isogenic strains of carrying either the shuttle vector pWM402 (right panel) or pINY1801 (a chimeric plasmid containing the cloned [surface exclusion protein, Sec10]) and [aggregation substance, Asc10] genes of pCFlO cloned into pWM402). The latter plasmid confers constitutive expression of Asc10 and Sec10 on its host cell ( ). The cells were preserved by a cryofixation procedure similar to that described by Erlandsen et al. ( ). Note the smooth appearance of the bacterial cell surface (right panel) and surface display of the plasmid-encoded proteins as discrete entities with well-defined structural features. Micrographs were kindly supplied by Dr. Stanley Erlandsen, University of Minnesota.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
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Figure 2

Production, release, and sensing of enterococcal peptides at the cell surface. In the recipient cell shown at the left, the lipoprotein signal peptide is processed by Eep in the cell membrane and the mature pheromone is secreted to the exterior, where it can diffuse into the extracellular medium or remain associated with the cell wall. The plasinid-containing donor cell depicted on the right can bind exogenous pheromone via the plasmid-encoded, peptide-specific binding protein (PrgZ/TraC) and use the chromosomal-encoded Opp system to import the peptide. This cell retains the ability to produce pheromone; however, the plasmid-encoded regulatory protein (PrgY/TraB) in the membrane reduces the endogenous pheromone, thus blocking an autocrine loop where endogenous pheromone could be immediately bound and reinternalized by the producing cell. In addition, the inhibitor peptide is secreted into the growth medium where it serves to neutralize any pheromone released by the same cell. The level of inhibitor secreted seems to be just enough to neutralize any endogenous peptide, while still allowing for exquisite sensitivity to exogenous pheromone.

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Figure 2

Production, release, and sensing of enterococcal peptides at the cell surface. In the recipient cell shown at the left, the lipoprotein signal peptide is processed by Eep in the cell membrane and the mature pheromone is secreted to the exterior, where it can diffuse into the extracellular medium or remain associated with the cell wall. The plasinid-containing donor cell depicted on the right can bind exogenous pheromone via the plasmid-encoded, peptide-specific binding protein (PrgZ/TraC) and use the chromosomal-encoded Opp system to import the peptide. This cell retains the ability to produce pheromone; however, the plasmid-encoded regulatory protein (PrgY/TraB) in the membrane reduces the endogenous pheromone, thus blocking an autocrine loop where endogenous pheromone could be immediately bound and reinternalized by the producing cell. In addition, the inhibitor peptide is secreted into the growth medium where it serves to neutralize any pheromone released by the same cell. The level of inhibitor secreted seems to be just enough to neutralize any endogenous peptide, while still allowing for exquisite sensitivity to exogenous pheromone.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
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Figure 3

Comparison of pADl, pCF10, pPDl, and pAM373 in regions that include determinants that relate to regulation of the pheromone response. ORFs indicated by similar colors are homologues in the different plasmids.

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Figure 3

Comparison of pADl, pCF10, pPDl, and pAM373 in regions that include determinants that relate to regulation of the pheromone response. ORFs indicated by similar colors are homologues in the different plasmids.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
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Figure 4

Model showing the control circuitry for the pADl and pCFlO pheromone response. P is a primary promoter that is active to some extent even in the uninduced state, giving rise to m3 (for pADl) and Qs (for pCF10), which are terminated at t/IRS1 (noted as 1 in the figure). Induction results in up-regulation from P, which gives rise to significant amounts of m3* (for pADl) and QL (for pCF10), which are terminated at t/IRS2 (noted as 2 in the figure). Induction also results in extension through t/IRS2 and into regions that include determinants that positively regulate conjugation genes. The positive regulators appear to differ significantly for the two plasmids. The short component of the speckled transcript represents mD (for pADl) and Qa (for pCF10), which are expressed under the Pa promoter and, at least in the case of pADl, enhance termination at t. Pa is also believed to govern expression of TraA/PrgX, which negatively regulates expression from P and is able to bind to pheromone. Expression from Pa is down-regulated upon induction.

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Image of Figure 4
Figure 4

Model showing the control circuitry for the pADl and pCFlO pheromone response. P is a primary promoter that is active to some extent even in the uninduced state, giving rise to m3 (for pADl) and Qs (for pCF10), which are terminated at t/IRS1 (noted as 1 in the figure). Induction results in up-regulation from P, which gives rise to significant amounts of m3* (for pADl) and QL (for pCF10), which are terminated at t/IRS2 (noted as 2 in the figure). Induction also results in extension through t/IRS2 and into regions that include determinants that positively regulate conjugation genes. The positive regulators appear to differ significantly for the two plasmids. The short component of the speckled transcript represents mD (for pADl) and Qa (for pCF10), which are expressed under the Pa promoter and, at least in the case of pADl, enhance termination at t. Pa is also believed to govern expression of TraA/PrgX, which negatively regulates expression from P and is able to bind to pheromone. Expression from Pa is down-regulated upon induction.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
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Figure 5

Map of the conjugative transposon showing ORFs believed to be related to conjugation. The transposon is approximately 18 kb in length. Apparent relationships with genes/ORFs in the database are noted.

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Image of Figure 5
Figure 5

Map of the conjugative transposon showing ORFs believed to be related to conjugation. The transposon is approximately 18 kb in length. Apparent relationships with genes/ORFs in the database are noted.

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
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Tables

Conjugation and Genetic Exchange in Enterococci
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Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
/content/10.1128/9781555817923.chap7.tab7-1
Table 1

Plasmids that encode a pheromone response

10.1128/9781555817923/tab7-1_thmb.gif
10.1128/9781555817923/tab7-1.gif
Generic image for table
Table 1

Plasmids that encode a pheromone response

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7
/content/10.1128/9781555817923.chap7.tab7-2
Table 2

Pheromone and inhibitor precursors

10.1128/9781555817923/tab7-2_thmb.gif
10.1128/9781555817923/tab7-2.gif
Generic image for table
Table 2

Pheromone and inhibitor precursors

Citation: Clewell D, Dunny G. 2002. Conjugation and Genetic Exchange in Enterococci, p 265-300. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch7

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