Replication termination and chromosome dimer resolution in the archaeon Sulfolobus solfataricus

Archaea of the genus Sulfolobus have a single‐circular chromosome with three replication origins. All three origins fire in every cell in every cell cycle. Thus, three pairs of replication forks converge and terminate in each replication cycle. Here, we report 2D gel analyses of the replication fork fusion zones located between origins. These indicate that replication termination involves stochastic fork collision. In bacteria, replication termination is linked to chromosome dimer resolution, a process that requires the XerC and D recombinases, FtsK and the chromosomal dif site. Sulfolobus encodes a single‐Xer homologue and its deletion gave rise to cells with aberrant DNA contents and increased volumes. Identification of the chromosomal dif site that binds Xer in vivo , and biochemical characterization of Xer/ dif recombination revealed that, in contrast to bacteria, dif is located outside the fork fusion zones. Therefore, it appears that replication termination and dimer resolution are temporally and spatially distinct processes in Sulfolobus . Eubacteria and archaea possess single‐circular chromosomes, yet some archaea resemble eukaryotes in using multiple origins and replication forks. Replication termination in Sulfolobus is found to occur by stochastic collision of these forks, and—unlike the situation in eubacteria—it is not linked to chromosome segregation.