Up to 60% of all cellular transcription in budding yeast occurs in the tandem arrays of rDNA. That transcription can prevent separation of sister chromatids during mitosis, according to Machín et al. (page 893).

The researchers started with Cdc14 phosphatase, which is needed to get cells out of mitosis. Cells mutant for Cdc14 could be forced out of mitosis by expressing a mitotic kinase inhibitor, but the majority of these cells died because they did not resolve their rDNA. But either reducing the amount of rDNA or inhibiting rDNA transcription was sufficient to allow these cells to survive and segregate rDNA correctly. Angelika Amon (MIT, Cambridge, MA) is reporting similar findingings in a paper in press.

There are two main theories to explain this surprising result. A combination of DNA replication and transcription may increase supercoiling and thus promote catenation. Or the proteins that do the cotranscriptional processing of rRNA may form a sticky mess that holds the sister chromatids together.

The stickiness may be disrupted by the chromatin-condensation complex condensin, which normally loads onto rDNA but fails to do so when Cdc14 is defective. Alternatively, condensin may contribute by reducing the amount of rDNA transcription. Old results suggested that rDNA transcription continued unabated through mitosis, but Machín et al. are retesting that assertion. Meanwhile, the relevant target for Cdc14's activity remains a mystery.