Yeast without Smc5-Smc6 don't finish replicating rDNA (green) and missegregate this DNA at mitosis.


Checkpoints exist to ensure the genome is accurately replicated each cell division. But yeast cells don't bother to check that replication is actually completed, report Jordi Torres-Rosell, Luis Aragón (MRC Clinical Sciences Centre, London, UK), and colleagues.

The team was not initially looking for a replication completion checkpoint. They were studying the function of a yeast complex called Smc5-Smc6 that had been suggested to promote DNA recombination and repair.

Budding yeast that lack Smc5-Smc6 do not survive more than a few cell cycles, so the team synchronized yeast cells in G1, knocked out the complex, and then observed the yeast over one cell cycle. S phase and mitosis appeared to be normal. In the subsequent interphase, however, the Rad53 DNA damage signal was activated. This damage response, the team discovered, was due to a failure of chromosomes to separate correctly at anaphase.

Thinking that this nondisjunction might be caused by a failure to resolve recombination events, the team knocked out critical recombination genes to see whether the problem was bypassed. Loss of recombination only slightly rescued the phenotype, however.

If Smc5-Smc6 was knocked out after S phase, but before metaphase, chromosomes segregated normally, showing that the complex is required during (or before) DNA replication. Loss of Smc5-Smc6 before S phase caused replication forks to persist into metaphase. Labeling of nascent DNA in these cells revealed that large regions of rDNA were still unreplicated. rDNA is a major binding site for Smc5-Smc6 and was also the main site of the nondisjunction.

Aragón suggests that Smc5-Smc6 might act as a chromatin structure modifier that allows the replication machinery to progress unhindered. Despite the slowed replication in the absence of Smc5-Smc6, the cells continued with cell division.

Yeast lack this replication completion checkpoint because they might not need it. There are many unused replication origins that can become active during late S phase, explains Aragón. Thus, it's likely that normal cells can easily replicate their DNA in time for metaphase.


Torres-Rosell, J., et al.