A Redefinition of the Mating Phenomenon in Bacteria

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145

Escherichia coli cells carrying the F sex factor are poor recipients in conjugation. This phenomenon is called surface exclusion. Two F cistrons, traS and traT, are independently responsible for part of the whole mechanism. The traS gene product reduces DNA transfer within stable mating aggregates. The tra T gene product, pTraT, results in a greatly reduced ability to form stable mating aggregates, and thus also leads to reduced DNA transfer within the cell population. pTraT is a 25,000-dalton protein incorporated into the cell envelope outer membrane. It is found in 29,000-84,000 copies per cell, depending on the plasmid expressing it. There is a parallel variation in recipient ability. Models for surface exclusion are discussed.Many bacterial sex factors not only code for the ability to act as donors and to transfer DNA, but they also reduce the ability of their hosts to act as recipients in conjugation with other donors. We refer to this phenomenon as surface exclusion. It is specific in that only conjugation with donors carrying closely related sex factors is inhibited (1). Surface exclusion is a result of active expression of the sex factor DNA because it can be alleviated, at least partially, either by growing cells into stationary phase (2) or by isolating mutants of the sex factor (3, 4). It has been argued (5) that surface exclusion is a result only of the inhibition of DNA transfer, without any effect on the formation of cell contacts. Our data here and elsewhere (6) do not support this view.We have analyzed bacterial conjugation and surface exclusion with the physical techniques of electronic particle analysis (Coulter counter) and electron microscopy (6-8). The results demonstrated that conjugation occurs in mating aggregates containing between 2 and 50 cells each, and that the aggregates are of variable composition in regard to their donor/recipient ratio (7). Furthermore, the formation of these mating aggregates involves several stages (8). F pili are necessary for the initial interactions, and then unstable wall-to-wall contacts occur. The wall-to-wall contacts are stabilized, and DNA transfer can then take place.

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Cell--cell interactions in conjugating Escherichia coli: role of traT protein in surface exclusion.

Achtman¹,

Kennedy²,

Skurray³

1977

178

145

“…15), and the existence of mating signals between recipients and donor cells has been inferred (16,17). There is also evidence that donor strains of Escherichia colh produce "recombination stimulating factors" (18) which affect recipients.…”

Section: Discussionmentioning

confidence: 99%

Induced cell aggregation and mating in Streptococcus faecalis: evidence for a bacterial sex pheromone.

Dunny¹,

Brown²,

Clewell³

1978

508

335

Recipient strains of Streptococcus faecalis produce a trypsin sensitive, heat resistant, nuclease resistant factor, designated clumping-inducing agent (CIA) which causes strains carrying certain conjugative plasmids to aggregate. RNA and protein synthesis but not DNA synthesis are required for aggregation to occur. Recipient filtrates that contain CIA activity also induce donors to mate at high frequencies. Introduction of a transferable plasmid into strains producing CIA dramatically reduces the amount of CIA activity produced by the strain but allows the strain to respond to exogenously added CIA. Our data suggest that CIA represents a bacterial sex hormone (pheromone).

“…F-pili, characteristic of Escherichia coli carrying the conjugative plasmid F, are helical polymers of one subunit, F-pilin (8). Conjugation commences when the tips of F-pili make contact with recipient cells (9,10). F-pili tips also bind filamentous DNA bacteriophages, such as M13, whereas icosohedral RNA bacteriophages such as R17 bind along the filament sides (11).…”

mentioning

confidence: 99%

F-pili dynamics by live-cell imaging

Clarke

Maddera

Harris

et al. 2008

130

139

Bacteria have evolved numerous mechanisms for cell-cell communication, many of which have important consequences for human health. Among these is conjugation, the direct transfer of DNA from one cell to another. For Gram-negative bacteria, conjugation requires thin, flexible filaments (conjugative pili) that are elaborated by DNA donor cells. The structure, function, and especially the dynamics of conjugative pili are poorly understood. Here, we have applied live-cell imaging to characterize the dynamics of F-pili (conjugative pili encoded by the F plasmid of Escherichia coli). We establish that F-pili normally undergo cycles of extension and retraction in the absence of any obvious triggering event, such as contact with a recipient cell. When made, such contacts are able to survive the shear forces felt by bacteria in liquid media. Our data emphasize the role of F-pilus flexibility both in efficiently sampling a large volume surrounding donor cells in liquid culture and in establishing and maintaining cell-cell contact. Additionally and unexpectedly, we infer that extension and retraction are accompanied by rotation about the long axis of the filament.conjugation ͉ conjugative pilus ͉ extension ͉ retraction A mong Gram-negative bacteria, conjugative DNA transfer is mediated by multicomponent type IV secretion systems commonly encoded by large plasmids (1, 2). By virtue of such activity, these plasmids rapidly spread within bacterial populations, contributing to the dissemination of antibiotic resistance (3) among other traits pertinent to human health (2, 4-6).A hallmark of Gram-negative bacterial cells capable of acting as DNA donors is the presence of surface filaments collectively designated conjugative pili (7). F-pili, characteristic of Escherichia coli carrying the conjugative plasmid F, are helical polymers of one subunit, F-pilin (8). Conjugation commences when the tips of F-pili make contact with recipient cells (9, 10). F-pili tips also bind filamentous DNA bacteriophages, such as M13, whereas icosohedral RNA bacteriophages such as R17 bind along the filament sides (11).Indirect evidence has suggested that F-pili are dynamic structures, capable of both extension and retraction. F-pilus retraction as an essential stage of DNA transfer was first proposed by Marvin and Hohn (12) and by Curtiss (13). Various lines of indirect evidence since then have supported the retraction hypothesis (14-18), although fundamental uncertainties remain. For example, retraction was initially regarded as a triggered event, occurring only when a recipient cell or bacteriophage bound to the filament tip (12). Later it was proposed that F-pili normally undergo cycles of extension and retraction (14,17,19). The idea that DNA can be conducted from donor to recipient through extended F-pili (10, 20) has also received experimental support (21,22).Here, we have applied laser-scanning confocal microscopy to study F-pilus dynamics with living cells. Our results clarify some of the uncertainties surrounding F-pilus function and sugge...