FtsK-Dependent Dimer Resolution on Multiple Chromosomes in the Pathogen Vibrio cholerae

Abstract

Unlike most bacteria, Vibrio cholerae harbors two distinct, nonhomologous circular chromosomes (chromosome I and II). Many features of chromosome II are plasmid-like, which raised questions concerning its chromosomal nature. Plasmid replication and segregation are generally not coordinated with the bacterial cell cycle, further calling into question the mechanisms ensuring the synchronous management of chromosome I and II. Maintenance of circular replicons requires the resolution of dimers created by homologous recombination events. In Escherichia coli, chromosome dimers are resolved by the addition of a crossover at a specific site, dif, by two tyrosine recombinases, XerC and XerD. The process is coordinated with cell division through the activity of a DNA translocase, FtsK. Many E. coli plasmids also use XerCD for dimer resolution. However, the process is FtsK-independent. The two chromosomes of the V. cholerae N16961 strain carry divergent dimer resolution sites, dif1 and dif2. Here, we show that V. cholerae FtsK controls the addition of a crossover at dif1 and dif2 by a common pair of Xer recombinases. In addition, we show that specific DNA motifs dictate its orientation of translocation, the distribution of these motifs on chromosome I and chromosome II supporting the idea that FtsK translocation serves to bring together the resolution sites carried by a dimer at the time of cell division. Taken together, these results suggest that the same FtsK-dependent mechanism coordinates dimer resolution with cell division for each of the two V. cholerae chromosomes. Chromosome II dimer resolution thus stands as a bona fide chromosomal process.

XerD-mediated FtsK-independent integration of TLCϕ into the Vibrio cholerae genome

“…S4). In similar experiments with tandem dif plasmids, dif/dif HJs were never detected, suggesting that they were more efficiently processed into product and/or back into substrate than attP TLC /dif1 HJs (5,20). Impeding XerD strand exchanges entirely abolished HJ and product formation, confirming that attP TLC /dif1 recombination was initiated by XerD catalysis (Fig.…”

“…In V. cholerae, as in most bacteria, the Xer machinery consists of two tyrosine recombinases, XerC and XerD. They act at a unique specific chromosomal site, dif, on each of the two circular chromosomes, chrI and chrII, of the bacterium (5). Integrative mobile elements exploiting Xer (IMEXs) carry a dif-like site on their circular genome, attP (3,4) (Fig.…”

Molecular keys of the tropism of integration of the cholera toxin phage

“…Instead, it has been shown that the El Tor variant of CTXϕ hijacks XerC and XerD, two host-encoded tyrosine recombinases that normally function to resolve chromosome dimers (5), to integrate at dif1, the dimer resolution site of the larger of the two V. cholerae chromosomes (6). Xer recombination sites consist of binding sites for XerC and XerD, separated by a 6-bp to 8-bp overlap region; strand exchanges occur at the border of this region (7).…”

Efficiency and specificity of CTXϕ chromosomal integration: dif makes all the difference

“…In Escherichia coli, XerCD proteins bind and catalyze recombination at homologous 28-bp dif sites, composed of two 12-bp binding sites for XerC and XerD separated by a 6-bp spacer or overlap region, which allows for XerC-XerD interactions that ensure stable synapsis (10,11). Because V. cholerae harbors two distinct, nonhomologous circular chromosomes (chromosome I and II) (12,13), two dif sites are present, dif1 in chromosome I and dif2 in chromosome II (9); similar to E. coli, the same FstKdependent mechanism coordinates dimer resolution on each chromosome with cell division (14). The dif1 site differs from dif2 at four polymorphic sites, one of which is located in the XerC binding site and the other three sites are located in the 6-bp spacer region (9,14).…”

“…Because V. cholerae harbors two distinct, nonhomologous circular chromosomes (chromosome I and II) (12,13), two dif sites are present, dif1 in chromosome I and dif2 in chromosome II (9); similar to E. coli, the same FstKdependent mechanism coordinates dimer resolution on each chromosome with cell division (14). The dif1 site differs from dif2 at four polymorphic sites, one of which is located in the XerC binding site and the other three sites are located in the 6-bp spacer region (9,14).…”