DNA Replication-Dependent Formation of Joint DNA Molecules in Physarum polycephalum

Abstract

Two-dimensional neutral/neutral agarose gel electrophoresis is used extensively to localize replication origins. This method resolves DNA structures containing replication forks. It also detects X-shaped recombination intermediates in meiotic cells, in the form of a typical vertical spike. Intriguingly, such a spike of joint DNA molecules is often detectable in replicating DNA from mitotic cells. Here, we used naturally synchronous DNA samples from Physarum polycephalum to demonstrate that postreplicative, DNA replication-dependent X-shaped DNA molecules are formed between sister chromatids. These molecules have physical properties reminiscent of Holliday junctions. Our results demonstrate frequent interactions between sister chromatids during a normal cell cycle and suggest a novel phase during DNA replication consisting of transient, joint DNA molecules formed on newly replicated DNA.

Alternate pathways involving Sgs1/Top3, Mus81/ Mms4, and Srs2 prevent formation of toxic recombination intermediates from single-stranded gaps created by DNA replication

“…In mammalian cells, sister chromatid exchanges occur spontaneously and likely reflect reciprocal exchanges occurring during replication (8,9). At the molecular level, X-shaped recombination intermediates, similar to HJs, are formed during mitotic replication at different loci of Physarum polycephalum (10) and in the Saccharomyces cerevisiae ribosomal DNA locus (11).…”

Role ofRAD52Epistasis Group Genes in Homologous Recombination and Double-Strand Break Repair

“…Branched-DNA molecules have been detected during mitotic S phase within the tandemly repeated rDNA locus, and formation of these intermediates is dependent on RAD52, suggesting that they correspond to recombination intermediates (433). Postreplicative, DNA replication-dependent X-shaped molecules have also been detected between sister chromatids in Physarum polycephalum (25). The X-forms are readily detected, indicating that they form at high frequency; this suggests that resolution of Holliday junctions could in fact be essential for chromosome segregation in eukaryotes.…”

The double life of Holliday junctions

“…Further, genetic observations seem to suggest that DSBs form rarely in mitosis and that DNA lesions accumulating during replication are probably one source for recombination events [10,11]. Replication-dependent sister chromatid junctions were indeed observed in diverse organisms (see for example [12][13][14]) and recently interhomologue junctions were found to accumulate at damaged replication forks in sgs1 mutants [15]. In some cases, these cruciform molecules resemble hemicatenanes, rather than HJs, and might originate during the bypass of DNA lesions through template switching.…”