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Unlinking chromosome catenanes in vivo by site-specific recombination

2007; Springer Nature; Volume: 26; Issue: 19 Linguagem: Inglês

10.1038/sj.emboj.7601849

1460-2075

Ian Grainge, Migena Bregu, Mariel Vázquez, Viknesh Sivanathan, Stephen C.Y. Ip, David J. Sherratt,

DNA and Nucleic Acid Chemistry

Article6 September 2007free access Unlinking chromosome catenanes in vivo by site-specific recombination Ian Grainge Ian Grainge Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Migena Bregu Migena Bregu Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Mariel Vazquez Mariel Vazquez Department of Mathematics, San Francisco State University, San Francisco, CA, USA Search for more papers by this author Viknesh Sivanathan Viknesh Sivanathan Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Stephen CY Ip Stephen CY Ip Department of Biochemistry, University of Oxford, Oxford, UKPresent address: Cancer Research UK, Clare Hall Laboratories, London Research Institute, Herts EN6 3LD, UK Search for more papers by this author David J Sherratt Corresponding Author David J Sherratt Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Ian Grainge Ian Grainge Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Migena Bregu Migena Bregu Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Mariel Vazquez Mariel Vazquez Department of Mathematics, San Francisco State University, San Francisco, CA, USA Search for more papers by this author Viknesh Sivanathan Viknesh Sivanathan Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Stephen CY Ip Stephen CY Ip Department of Biochemistry, University of Oxford, Oxford, UKPresent address: Cancer Research UK, Clare Hall Laboratories, London Research Institute, Herts EN6 3LD, UK Search for more papers by this author David J Sherratt Corresponding Author David J Sherratt Department of Biochemistry, University of Oxford, Oxford, UK Search for more papers by this author Author Information Ian Grainge1,‡, Migena Bregu1,‡, Mariel Vazquez2, Viknesh Sivanathan1, Stephen CY Ip1 and David J Sherratt 1 1Department of Biochemistry, University of Oxford, Oxford, UK 2Department of Mathematics, San Francisco State University, San Francisco, CA, USA ‡These authors contributed equally to this work *Corresponding author. Division of Molecular Genetics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK. Tel.: +44 1865 275296; Fax: +44 1865 275297; E-mail: [email protected] The EMBO Journal (2007)26:4228-4238https://doi.org/10.1038/sj.emboj.7601849 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info A challenge for chromosome segregation in all domains of life is the formation of catenated progeny chromosomes, which arise during replication as a consequence of the interwound strands of the DNA double helix. Topoisomerases play a key role in DNA unlinking both during and at the completion of replication. Here we report that chromosome unlinking can instead be accomplished by multiple rounds of site-specific recombination. We show that step-wise, site-specific recombination by XerCD-dif or Cre-loxP can unlink bacterial chromosomes in vivo, in reactions that require KOPS-guided DNA translocation by FtsK. Furthermore, we show that overexpression of a cytoplasmic FtsK derivative is sufficient to allow chromosome unlinking by XerCD-dif recombination when either subunit of TopoIV is inactivated. We conclude that FtsK acts in vivo to simplify chromosomal topology as Xer recombination interconverts monomeric and dimeric chromosomes. Introduction In the first descriptions of DNA structure, Watson and Crick (1953) identified the potential consequences of the interwound strands of the DNA double helix. The two strands of the duplex must be separated in order to be copied, but they are wrapped around each other once per helical turn, an average of ∼420 000 times in the supercoiled 4.6 Mbp Escherichia coli chromosome. Every one of these links must be removed in order to separate the two replicated chromosomes. Strand separation during replication pushes the helical twists into the region ahead of the replication fork, leading to an accumulation of positive supercoiling, which must be removed if DNA synthesis is to progress. This task is accomplished primarily by the essential type II topoisomerase, DNA gyrase. If any positive supercoiling diffuses behind the replication fork, interwinding of the two newly replicated sister duplexes in a partially replicated chromosome will occur, forming precatenanes (which become catenanes when replication is complete). In E. coli, a second type II topoisomerase, TopoIV, a heterotetramer of ParC and ParE, normally plays an essential role in chromosome segregation by acting preferentially on precatenanes or catenanes, particularly in the late stages of replication, when DNA gyrase may not be able to access the small region of unreplicated DNA between the converging replication forks (reviewed in (Espeli and Marians, 2004; Schvartzman and Stasiak, 2004)). Both linear and circular chromosomes are locally catenated at the end of replication. However, in the case of circular chromosomes, crossing over by homologous recombination at stalled or broken replication forks generates dimeric circular chromosomes (Sherratt et al, 1995) in which any pre-catenation crossings would be converted to crossings in the knotted dimer. Dimeric chromosomes are converted to monomers by XerCD site-specific recombination acting at the recombination site, dif, located in the replication termination region (reviewed in Barre and Sherratt, 2005). FtsK, a multifunctional protein that coordinates chromosome segregation and cell division, plays an essential role in dimer resolution. FtsK localises to the septum during the late stages of chromosome replication through its N-terminal domain (FtsKN) which functions in cytokinesis (Liu et al, 1998; Wang et al, 2005). The C-terminal domain of FtsK (FtsKC) uses ATP-dependent DNA translocation to synapse two sister dif sites in a conformation that is activated for dimer resolution by XerCD (Steiner et al, 1999; Aussel et al, 2002). Faithful and efficient chromosome segregation requires that the processes of decatenation and dimer resolution are coordinated in time and space. Consistent with this, TopoIV associates with the dif region (Hojgaard et al, 1999) and interacts with FtsK to stimulate TopoIV decatenation (Espeli et al, 2003a). In E. coli, TopoIV, FtsK, the dif site and the replication machinery all appear to be associated with midcell in the late stages of chromosome replication (Espeli and Marians, 2004; Wang et al, 2005). We have previously shown that XerCD-dif-FtsK recombination can unlink catenanes formed by site-specific recombination in vitro (Ip et al, 2003). Here we extend that result by showing that XerCD-dif-FtsK recombination can unlink in vitro the range of replication catenanes accumulated in vivo during TopoIV impairment. We also show mathematically that efficient unlinking must occur in a stepwise fashion with catenanes and knotted dimers being interconverted at each step, a mechanism supported by experiments. We provide support for the direct involvement of XerCD-dif-FtsK recombination in in vivo chromosome unlinking when TopoIV activity is compromised. Further, Cre-loxP site-specific recombination can substitute for XerCD-dif recombination in chromosome unlinking, both reactions requiring KOPS-directed translocation by FtsK. Finally we show that the expression of cytoplasmic FtsK is sufficient to allow chromosome segregation in both parEts and parCts backgrounds independently of the previously identified suppressor, DnaX(γ). Taken together, this is compelling evidence that site-specific recombination can act to unlink chromosomes following replication independently of TopoIV, the major cellular decatenase. Results XerCD-dif-FtsK recombination unlinks plasmid replication catenanes formed in vivo XerCD-dif-FtsK recombination converts unknotted supercoiled DNA dimers exclusively into monomeric free circles. This requires that ATP hydrolysis-dependent FtsK translocation along DNA directs the formation of simply synapsed dif sites, whose recombination leads to topologically simple products (Aussel et al, 2002). This selectivity of XerCD-FtsK recombination for topologically simple dif synaptic complexes suggested that such recombination might unlink catenated DNA, a prediction that was confirmed experimentally using simple catenanes with either parallel or antiparallel dif sites (Ip et al, 2003). To explore whether XerCD-dif-FtsK recombination might be directly involved in vivo in the unlinking of newly replicated chromosomal DNA, we tested first whether this recombination can unlink the range of interlinked products formed by replication in vivo. To obtain these, we used a temperature-sensitive parCts mutant of TopoIV (Kato et al, 1988), harbouring a multicopy plasmid with a dif site (pUC-dif). Newly replicated dif-containing plasmid catenanes accumulated in vivo within minutes after the switch of parCts cells containing pUC-dif to 42°C (Adams et al, 1992) (Figure 1A). Because dif-plasmids are not good in vivo substrates for XerCD-FtsK recombination unless a cytoplasmic form of FtsK is overexpressed (unpublished data), we did not expect that such catenanes would be efficiently unlinked in vivo by XerCD-dif recombination. After isolation of supercoiled plasmid DNA from such a culture, the catenane band was gel extracted and shown to contain molecules with 2–14 catenation links after DNaseI nicking (Figure 1B, lane 1). A low level of dimeric knots was also present. The supercoiled replication catenane population was reacted in vitro with XerCD and FtsK50C, a biochemically active form of FtsK (Aussel et al, 2002; Ip et al, 2003). In the presence of ATP, the catenane population was efficiently converted to free circles in a 30 min 37°C reaction (lane 6). This reaction was dependent on ATP hydrolysis, since no reaction was observed with the nonhydrolysable analogue, AMPPNP or poorly hydrolysable ATPγS (not shown). Figure 1.Unlinking of replication catenanes by Xer recombination. (A) The strategy used to isolate plasmid replication catenanes. parCts cells harbouring a pUC-dif plasmid were grown at 30°C and then were subjected to a 30 min 42°C temperature shift. The supercoiled catenated dif-plasmid DNA (sc cats) was gel-extracted. In this and subsequent figures, double-stranded DNA is represented by a single line, and dif sites by a triangle. (B) Unlinking by XerCD-dif-FtsK50C recombination of replication pUC-dif catenanes formed in vivo. Product topology is revealed after DNaseI nicking (all lanes). Unlinked monomers are labelled as free circle products: oc, open circle; and lin, linear. Left panel: time course of XerCD-dif-FtsK50C recombination of replication catenanes at 25°C, alongside a 37°C control reaction. Dimeric DNA knots and unknotted dimeric DNA (oc dimer) appear as transient reaction intermediates in the course of catenane unlinking. Right panel: XerCD-dif-FtsK50C conversion of DNA dimer torus knots to free circles. Download figure Download PowerPoint Initial considerations suggested that the unravelling of replication catenanes to free circles by XerCD-FtsK50C would proceed in a stepwise manner with one linking node removed per recombination event, with catenated monomers and knotted dimers being interconverted at each recombination step (Ip et al, 2003) (also see Figure 2D). In an attempt to slow the reaction and resolve these intermediates, a time course of unlinking was carried out at 25°C. Examination of the unlinking patterns of replication catenanes at 25°C was consistent with this stepwise unlinking (Figure 1B; lanes 2–5). In particular, knotted dimers appeared as transient recombination intermediates as the reaction progressed. While the majority of transient knotted forms may be difficult to discern, the appearance of the five-noded knot is the most apparent. Figure 2.Tangle methodology for stepwise unlinking by XerCD recombination at dif. The three possible topological mechanisms, which eventually yield unlinked products, are shown for the reaction from a RH 6-crossing catenane to a (+)5-torus knot (A–C). The tangle notation is presented in the framed figure. O, P and R are tangles, one round of recombination at dif by XerCD-FtsK is represented by a system of two tangle equations: N (O+P)=K1, N (O+R)=K2. Here K1 is a RH 6-cat with parallel sites, and the product K2 is assumed a knot or catenane of six or less crossings. The pre- and post-recombination synapse are trapped inside P and R, respectively. The tangle O encloses all nontrivial topology outside the synapse. Panels A, B and C illustrate the only three plausible solutions to this system of equations. The three proposed solution mechanisms recombine the 6-catenane into the RH 5-crossing torus knot; repeated rounds of recombination result in a stepwise unlinking of the substrate (D). In the leftmost diagrams viewing the synapse along the light blue axis converts A into B. Similarly a left-handed 180° rotation of the synapse in A around the black axis converts this to C. Therefore, these three solutions correspond to different projections of the same three-dimensional object. Note also that the three products shown in the right-hand panel are all the same five-noded knot product. Download figure Download PowerPoint Catenane unlinking through DNA knot intermediates demands that XerCD-FtsK50C must be efficient at unlinking both DNA knots and catenanes. To demonstrate this, XerCD-FtsK50C recombination was assayed on torus knots that have equivalent topology and arrangement of dif sites to knot intermediates derived from replication catenanes. A population of supercoiled right-handed torus knots with dif sites in direct repeat was made by Cre-loxP site-specific recombination of a plasmid containing directly repeated dif sites and inverted loxP sites. A population of such supercoiled knots containing three-, five-, seven- and nine nodes was recombined to free circles when treated with XerCD-FtsK50C and ATP (Figure 1B; lanes 7–9). Note that this is a different substrate from the catenanes used in the left-hand panel (lanes 1–6), and so yields two nicked circular products of different sizes. The demonstration that XerCD-FtsK50C recombination in vitro efficiently unlinks the range of replication catenanes accumulated in vivo after TopoIV inactivation strengthens the hypothesis that XerCD-FtsK recombination at dif may assist TopoIV in unlinking newly replicated DNA. Tangle analysis supports a stepwise unlinking mechanism by XerCD-FtsK We applied the tangle method, under assumptions suited for the Xer system, to assess rigorously whether stepwise unlinking is the only pathway for unlinking of replication catenanes by XerCD (Sumners et al, 1995). The software TangleSolve (Saka and Vazquez, 2002) was used to compute the topological mechanisms producing prime knots or catenanes with six or less crossings from a right-hand (RH) 6-cat substrate with sites in parallel orientation. There were only three plausible mechanisms with the correct site orientation, which are interpreted as three different projections of the same three-dimensional object (Figure 2). In other words, a single three-dimensional pathway satisfies the model. Iterating any of these mechanisms results in a stepwise unlinking taking the RH 6-cat to (+)5-torus knot, to RH 4-cat, to (+) trefoil, to 2-cat, to unknot, to free circles. Other pathways producing more complex intermediates containing six or more crossings were also considered, however the stepwise reduction in complexity of replication (torus) catenanes, via (torus) knotted intermediates, was the only route to efficiently produce free circles (Figure 2 and Supplementary data). In summary, rigorous mathematical analysis supports the experimental evidence that unlinking of catenated newly replicated DNA by XerCD-dif recombination occurs by a stepwise reaction involving interconversion of catenated and knotted intermediates, with the topology being simplified at each step, until two free circles are produced. The rescue of TopoIV temperature sensitivity by DnaX overexpression is dependent on XerCD-dif-FtsK recombination Given that XerCD-dif-FtsK recombination in vitro unlinks plasmid replication catenanes, we addressed whether this recombination can remove catenation links from the chromosome at the completion of DNA replication, in addition to resolving chromosome dimers at dif. XerCD-dif-FtsK action is confined to the replication termination region (ter), at a time when TopoIV, FtsK, the replisome and ter are all localised to midcell and when completion of both DNA unlinking and dimer resolution must be achieved for chromosome segregation to occur successfully (Espeli et al, 2003b; Wang et al, 2005). TopoIV associates with dif and with the replisome at a time when they are all positioned in the midcell region and appears to have its unlinking activity stimulated by FtsK (Hojgaard et al, 1999; Espeli et al, 2003a, 2003b). To assess the direct involvement of XerCD-dif-FtsK recombination in chromosome unlinking, TopoIV decatenation activity had to be impaired. Therefore, we exploited the fact that the temperature sensitivity of a parEts strain is suppressed by overexpressing the DnaX γ clamp loader (Levine and Marians, 1998). This suppression, which was proposed to arise by the stabilisation of ParEts by γ, is also dependent on FtsK, a result used to support experimental evidence for a physical interaction between FtsK and TopoIV (Espeli et al, 2003a). In light of the unlinking activity displayed by XerCD-FtsK50C in vitro, we addressed whether suppression of parEts temperature sensitivity by γ overexpression is a direct consequence of chromosomal decatenation by XerCD-FtsK recombination at chromosomal dif. Strains containing parEts in combination with mutant alleles of xerD, xerC, xerCYF and dif were transformed with plasmid pLex-dnaX(γ) that overexpresses γ in the presence of IPTG (Espeli et al, 2003b). To monitor the ability of γ overexpression to support survival at restrictive temperatures, growth of these strains was examined in liquid media at the permissive and nonpermissive temperatures in the presence and absence of γ overexpression (Figure 3A and B). All strains grew well at 30°, with similar growth rates, irrespective of the presence of IPTG. As expected from previous experiments, γ overexpression suppressed the temperature sensitivity of parEts at 43° as judged by A600 increase (Figure 3A) and by colony formation (Figure 3B). However, overexpression of γ did not restore the growth of parEts strains that did not produce XerC or XerD, or that contained a catalytic xerCYF mutation. As reported previously, deletion of dif also led to a lack of suppression of temperature sensitivity by γ overexpression (Espeli et al, 2003a). The inability of γ overexpression to support the continued growth of the parEts xerCYF strain leads us to favour a model in which the failure to suppress is due to the lack of Xer recombination at dif, since XerCYF forms a functional synapse, capable of strand exchange by XerD (Bregu et al, 2002), and is therefore likely to interact functionally with its partners. Figure 3.Suppression of parEts temperature sensitivity by γ overexpression is dependent on XerCD-dif recombination. (A) Typical growth curves at 30 or 43°C for strains that are isogenic except for the indicated mutations, with or without IPTG to induce γ overexpression, as indicated. Each contained the γ overexpression plasmid, pLexA-dnaX(γ). To the right are histograms showing the mean values +/− 1 standard deviation of A600 of five independent cultures for each strain. (B) Colony-forming units for the strains at 30°C (left panel) and 43°C with IPTG (middle panel) and 43° without IPTG (right panel). Cultures were grown to an A600∼0.6, serially diluted (as indicated), plated and grown in the indicated conditions. (C) Typical growth curves at 30°C or 43°C, with and without IPTG, of the indicated recA strains. Download figure Download PowerPoint XerCD-dif-FtsK recombination rescues parEts cells in the absence of dimer formation by homologous recombination To rule out the possibility that the failure of XerCD-dif recombination-deficient cells to suppress the temperature sensitivity of parEts cells during γ overexpression was a consequence of an inability to resolve chromosome dimers, an recA mutation was introduced into the test strains and growth was examined (Figure 3C). γ overexpression supported the 43°C growth of parEts recA strains proficient for XerCD recombination at dif, but not in those deficient for XerCD-dif recombination. Therefore, XerCD-dif recombination is needed to suppress ParEts temperature sensitivity by γ overexpression in the absence of chromosome dimers formed by homologous recombination. Chromosome separation is impaired in parEts γ-overexpressing cells at 42°C when Xer recombination is compromised The role of Xer recombination in the rescue of parEts by overexpression of γ was examined by flow cytometry. First it had to be established whether TopoIV impairment affects ongoing replication. To test this, strains were grown in minimal media at the permissive temperature and then treated with cephalexin and rifampicin, replication 'run-out' conditions, at 30 and 42°C, where replication is completed, but replication re-initiation and cell division are blocked. The analysis revealed almost identical profiles for the parEts strain at both temperatures, with most cells having two chromosome equivalents at the end of the run-out. This shows that ongoing replication of the bulk of the chromosome continues during TopoIV impairment (Figure 4A), consistent with an earlier report (Espeli et al, 2003b). Any effect upon very late stages of replication cannot be assessed by this technique. The xerC and recA derivatives of the parEts strain also showed the ability to complete replication in run-out conditions at nonpermissive temperature, as did impairment of TopoIV by a parCts mutation (data not shown). Figure 4.Chromosome segregation requires an active site-specific recombination system at the terminus. (A) Flow cytometry histograms of replication run-out at 30°C compared to run-out at 42°C of a parEts strain initially grown at 30°C. Chromosome equivalents are indicated above each peak. DNA staining is with syto-16. (B) Flow cytometry histograms showing replication run-out of strains following growth for 3 h at 42°C; the left-hand set of panels shows the Xer+ strains, and the right-hand panels are the corresponding xerC derivative strains. DNA staining is with syto-16. Chromosome content was determined by comparison of multiple experiments with both syto-16 and propidium iodide staining and is marked above each peak. Individual peaks are not observed in the strains overexpressing γ. (C) Micrographs of DAPI-stained cells grown at 42°C for 3 h. Fields from two independent experiments are shown. Note the absence of segregated nucleoids in the xerC derivative strain (right hand panels) as well as numerous anucleate cells: classical Par− phenotype. The bar represents 10 μm. (D) Micrographs of DAPI-stained parEts xerC cells where the dif site is replaced by loxP, and both γ and the Cre recombinase are expressed, grown in LB at 42°C for 3 h. The cells are filamentous but have well separated nucleoids. Download figure Download PowerPoint Having established that ParEts inactivation does not stop ongoing chromosome replication, the effects of ParEts inactivation on chromosome segregation were compared in Xer+and Xer− backgrounds. Cells grown in minimal media at the permissive temperature were shifted to 42°C for 3 h, followed by a further 3 h at 42°C with cephalexin and rifampicin, prior to analysis by flow cytometry (Figure 4B). With all the strains tested, growth and run-out at 30°C showed very little variation between the Xer+ and Xer− strains (data not shown). After growth at nonpermissive temperature, replication run-out revealed a larger proportion of cells with higher chromosome content in the parEts xerC strain than in parEts Xer+. The patterns observed are consistent with a failure to segregate chromosomes in both strains, but seemingly worse in the Xer− background. A similar trend toward higher chromosome content in an xerC derivative was also seen with a recA background, showing that the higher chromosome content is not related to a failure to resolve chromosome dimers. To examine the effect of γ overexpression, a parEts recA strain and its xerC derivative were used to remove any complication of chromosome dimer formation. After 3 h growth at 42°C, both these strains were filamentous. The parEts recA cells overexpressing γ went from a mean length of 5.0 μm (σ=1.4, n=50) at 30° to a mean length of 19.4 μm (σ=7.9, n=64) at 42°C. A similar difference was seen with the xerC derivative. This increase in length is in the range expected if cell division stopped at 42°C, but the cells continued growing at a similar rate; 3 h growth reflects about two doublings in this growth media. Without γ overexpression, the parEts cells rarely achieved this filament length, because of cessation of cell growth and/or ongoing division to produce the many anucleate cells observed. The propensity to filament masks any possible effects of Xer mutation on the chromosome content of γ-overexpressing cells, as judged by chromosome content during flow cytometry; presence or absence of homologous recombination had little overall effect on the chromosome content (Figure 4B). However, when DAPI-stained cells were examined microscopically, it was apparent that the filaments produced by γ overexpression contained smaller well-separated nucleoids in an Xer+ background, but had unseparated DNA in an Xer− background (Figure 4C). The average length of nucleoids in γ-overexpressing parEts recA cells was 3.05 μm (σ=0.94, N=89), whereas in the xerC derivative the average length was 6.39 μm (σ=1.78, N=56), indicative of a failure to segregate the chromosome in the absence of Xer recombination. For comparison, the nucleoid lengths at 30°C were 3.10 μm (σ=0.65, N=100) and 3.16 μm (σ=0.82, N=100), respectively, for the parE recA and parE recA xerC strains overexpressing γ. The finding that in the absence of γ overexpression an Xer− derivative accumulates higher chromosome content than an Xer+ cell implies that Xer recombination is going some way to allow segregation in parEts cells at 42°C, but that without γ overexpression, it is not sufficient to allow cell survival. It is plausible that DnaX overexpression allows more time for the cells to decatenate their chromosomes by recombination, leading to inhibition of cell division in the short term, but viability in the long term. Cre-loxP site-specific recombination can substitute for XerCD-dif in γ-mediated suppression of the parEts phenotype Suppression of the parEts phenotype by overexpression of γ is thus dependent on XerCD, FtsK and dif. If this suppression is through iterated rounds of site-specific recombination, which reduce the catenation crossings of the chromosomes until they are unlinked from each other, then it might be possible that another recombination system could substitute for XerCD-dif in the terminus. To test this, a parEts strain in which the dif site was replaced by loxP was analysed. FtsK-dependent replacement of XerCD-dif by Cre-loxP for chromosome dimer resolution has been demonstrated previously (Leslie and Sherratt, 1995; Capiaux et al, 2002). Cells expressing a low constitutive level of the Cre recombinase and containing pLex-dnaX(γ) were grown on LB plates with either glucose or IPTG at 30 or 42°C. Cells grew well at 30°C, but none survived to give colonies at 42° with glucose, or when the Cre expression vector was absent. Conversely, at the nonpermissive temperature in the presence of IPTG, growth was observed with a mixture of several large colonies, as well as many small colonies. When re-streaked at 42°C, the cells again showed variable growth, with some clones again giving variable-sized colonies and some giving larger colonies. An xerC derivative of the same strain containing both the Cre and γ expression vectors exhibited the same behaviour, thereby demonstrating that suppression of the temperature sensitivity is XerC-independent when Cre-loxP recombination can occur. Individual large and small clones on the 42°C plates were grown in liquid culture at 42°C in the presence of IPTG. Dilutions were plated onto LB agar with IPTG, alongside a parEts+pLex-dnaX(γ) control. All cultures yielded similar numbers of viable colonies upon growth at either 30 or 42°C. Also, samples were DAPI stained and examined microscopically (Figure 4D). The filamentous cells (as previously observed when γ was overexpressed at 42°C), had well-segregated nucleoids along their length, similar to those in a strain competent for XerCD-dif recombination, confirming that unlinking of chromosomes was occurring within the cell filaments. We conclude that iterative Cre-loxP recombination can substitute for XerCD-dif recombination in unlinking chromosomes, thereby rescuing the ParEts phenotype when γ is overexpressed. The failure of the parEts loxPΔdif cells expressing γ and Cre to give uniform growth at 42°C may be related to the requirement to maintain two separate plasmids within each cell. When TopoIV is impaired, plasmid replication catenanes accumulate (Zech