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Genetically modified microorganisms (GMMs) used in the biotechnological industry are normally kept physically closed off from their surroundings. The strains used are attenuated and will not survive very long if they escape into the environment. In recent years there has been a growing interest in the deliberate release of GMMs into the environment. GMMs suitable for release could, for example, be used for bioremediation of polluted soils, for biocontrol of fungicidal and insecticidal pests in agriculture, or as live vaccines in biomedicine. Upon release, such strains must be able to proliferate and compete with the indigenous strains present. However, to ensure safety, uncontrolled spread of GMMs into the environment must be prevented. Safety can be achieved through biological containment if the GMMs self-destruct by expression of killing genes after fulfilling their jobs. Several bacterial toxins are good candidates for use in bacterial containment systems, including membrane-destabilizing or pore-forming proteins (4, 15) and enzymes attacking the genetic material of the cell (1, 2, 7). The design and applications of active biological containment systems are reviewed and discussed in several publications (8, 9, 12, 13).

Recently, relBE, members of a new toxin-antitoxin gene family, have been found in Escherichia coli (5). To date, relBE homologues have been identified in a broad range of both gram-negative and gram-positive bacteria and in archaea (5, 6). The relE gene encodes a small (11-kDa) protein that is extremely toxic to bacterial cells, and the relB gene encodes an antitoxin of similar size that counteracts the cell killing activity of the RelE toxin (5, 6). The specific molecular targets of the RelE protein, as well as the physiological role of the RelE-RelB toxin-antitoxin system in bacteria, is still speculative (3, 5). So far, no relBE homologues have been found in eukaryotes.

In the study described here, we analyzed whether this toxin-antitoxin gene system also could be used to control proliferation of eukaryotic cells. For this purpose, we used Saccharomyces cerevisiae as a general model for eukaryotes, specifically fungi. We showed that expression of relE strongly inhibits the growth of yeast cells and that the products of relE and relB interact.

Strains and media. E. coli TOP10 (Invitrogen) was routinely used during vector constructions. The bacteria were maintained and grown in Luria-Bertani medium (14) supplemented with ampicillin (100 µg/ml). The yeast strain used was S. cerevisiae 281288DIV-36 (MAT a his4-15 ura3-52 trp1; Y493 from our laboratory collection). Yeast strains were transformed as described previously (11). Transformed yeast cells were grown in liquid or solid SCura (synthetic complete medium without uracil and with 2% glucose), SCura+gal (synthetic complete medium without uracil and with 2% galactose), SCuramet (synthetic complete medium without uracil and methionine and with 2% glucose), or SCuramet+gal (synthetic complete medium without uracil and methionine and with 2% galactose) (11). When necessary, the media were solidified by addition of agar to 2% (wt/vol).

Construction of a vector for expression of the RelE toxin in S. cerevisiae. The DNA manipulations were performed according to standard methods (14). All PCR amplifications (20 cycles consisting of 40 s of denaturation, 40 s of annealing, and 1 min of extension) were performed with Vent DNA polymerase (New England Biolabs) using a PTC-100 thermocycler (MJ Research Inc.). After agarose gel electrophoresis, amplified fragments were isolated using a DNA purification kit from Qiagen.

Construction of a vector for expression of the RelE-RelB toxin-antitoxin in S. cerevisiae. A modified version of the pYES2 expression vector was constructed by removing the GAL1 promoter and inserting the methionine (MET25) promoter from S. cerevisiae. P_MET25 was amplified from pYC012 (10) by PCR using the forward primer 5'-AGACTAGTCCCGGGCTTAATTAAATAATATAC-3' and the reverse primer 5'-AGACTAGTGGATCCTGTATGGATGGGGGTA-3'. SpeI restriction enzyme recognition sites added at the 5' ends of both primers are underlined, and the BamHI site added in the reverse primer is in italics. P_GAL1 was removed from pYES2 as an SpeI fragment, and the SpeI-digested PCR fragment was inserted into the SpeI site of this vector, yielding plasmid pPK908. Twenty-three nucleotides at the 5' end of the PCR product were removed because of the presence of an SpeI restriction site in P_MET25. This deletion did not appear to have any significant effect on the promoter activity. The correct orientation of the inserted promoter was verified by restriction enzyme digestions.

Expression of relE in S. cerevisiae. Yeast cells transformed with either plasmid pPK727 (containing relE under the control of the GAL1 promoter) or the pYES2 vector were grown overnight in SCura, then diluted 1:100 in SCura or SCura+gal. Growth experiments were performed in 250-ml bottles with 20-ml cultures at 28°C with heavy agitation. The presence of glucose repressed the relE gene. Expression of relE from the GAL1 promoter in pPK727 was induced by galactose when the SCura was replaced by SCura+gal. A clear inhibitory effect on cell growth in SCura+gal was observed for yeast cells containing the pPK727 plasmid, whereas no such inhibition was observed for yeast cells containing the empty pYES2 vector (Fig. 1). The same inhibitory effect was also observed when pPK727 was introduced into other yeast strains (data not shown).

View larger version (17K): [in this window] [in a new window]   FIG. 1.   Effect of relE expression, under the regulation of the GAL1 promoter, on cell growth of S. cerevisiae. Cells harboring pPK727 were grown in SCura () or SCura+gal (), and cells harboring pYES2 as a control were grown in SCura () or SCura+gal ().

Expression of relE-relB in S. cerevisiae. Yeast cells transformed with plasmid pPK1006 (containing relE under the control of the GAL1 promoter and relB under the control of the MET25 promoter) were studied to see if there were interactions between the two proteins. The overnight culture was diluted 1:100 in four different growth media, SCura, SCuramet, SCura+gal, and SCuramet+gal. Growth experiments were performed in 250-ml bottles with 20-ml cultures at 28°C with heavy agitation. Expression of relE was induced by galactose (in SCura+gal and SCuramet+gal) but repressed by the presence of glucose. Expression of relB was induced in the absence of methionine (in SCuramet and SCuramet+gal) but repressed in the presence of methionine. Cells grown in SCuramet+gal showed higher growth rates than cells grown in the corresponding medium supplemented with methionine (Table 1). These results indicate that expression of relB counteracts the growth-inhibiting effect of the relE gene to some extent. A difference in doubling time was also observed for cells grown in SCura or SCuramet and can be explained by the fact that small amounts of the RelE toxin are synthesized also in the absence of galactose, due to a leaky activity of the uninduced GAL1 promoter. This growth-inhibiting effect was counteracted by the action of the RelB protein in SCuramet (Table 1). The relatively small difference in doubling time between yeast cells grown in SCuramet+gal and those grown in SCura+gal suggested that either the level of relB expression was too low to fully counteract the toxic effect of the induced relE expression or the RelB-RelE interaction in the yeast cell was very weak.

Conclusions. The data presented here clearly demonstrate that expression of the bacterial relE gene is also toxic to S. cerevisiae cells. The fact that the growth rates of relB-expressing cells are higher than those of non-relB-expressing cells suggests that both gene products function in S. cerevisiae. Thus, bacterial toxin-antitoxin systems can also be applied in eukaryotic cells. It is likely, even if so far it cannot be demonstrated directly, that the actions of the two genes occur at the protein level. The absence of a complete counteraction of the activity of relE by relB in SCuramet+gal could be partially reversed if a stronger promoter controls the relB gene, leading to overproduction of antitoxin compared to toxin. So far, it is not known whether several RelB molecules are required to counteract a single RelE molecule. Further experiments, in which the expression level of RelB is substantially higher than that of RelE, might demonstrate a total counteraction of RelE by RelB. Such data would support the hypothesis that the molecular targets for the RelE and RelB proteins in bacteria and S. cerevisiae are the same.