Cohesins are known to bind a heterochromatin protein at centromeres, but previous reports of cohesin localization on chromosome arms were incomplete. The authors did a comprehensive survey of four budding yeast chromosomes and two fission yeast chromosomes using chromatin immunoprecipitation. In budding yeast, 91% of cohesin sites were between converging genes, and cohesins were bound to 84% of the 328 convergent intergene regions.
Earl Glynn, Jennifer Gerton (Stowers Institute, Kansas City, MO) and colleagues gathered similar data for budding yeast. They suggest that transcription displaces cohesin from the DNA of active genes, and that the chromatin in convergent intergenic regions may be a stickier substrate for cohesin.
But Uhlmann's group favors the sliding model. Looking earlier, they saw that cohesins initially load at sites defined by the loading protein Scc2. These sites are most often in highly transcribed regions, but soon after loading the cohesin moves toward the intergenic regions. This would get the cohesin away from the loading proteins, which may have an opening activity that can also promote unloading.
Cohesin that was seen in the middle of several dormant genes later moved downstream when the genes were turned on during either meiosis or heat shock. Passive pushing is conceivable, says Uhlmann, because proteins bound to RNA polymerase and its nascent transcript “make the transcription machinery quite enormous.” Pushing would prevent transcripts from getting stuck in the rings. The replication machinery may, however, slip through the cohesin rings, which are loaded well before DNA replication starts. ▪