St. Jude study yields secrets of chromosom...( these events individually. By sep...)

By separating the Ago1 component of the RITS complex away from the Chp1-Tas3 components, rather than completely removing Ago1 from the cell, the researchers were able to generate yeast that could still maintain heterochromatin it had already established. These yeast cells had normal chromosome movement during cell division.

The investigators then showed that the same yeast cells also require the continuous presence of Clr4 to initially assemble normal centromeric heterochromatin. Specifically, the researchers used these yeast cells to show that the establishment of centromeric heterochromatin requires Clr4 to methylate histones at the centromere. The methylated centromere then attracts the RITS complex to this site. The St. Jude researchers work also suggests that the siRNA is not so important for this first step, but does play an important role in propagating and maintaining centromeric heterochromatin.

Until we did this study, it was virtually impossible to figure out which molecular events were specifically required for the two different processes of establishing and maintaining centromeric heterochromatin, Partridge said. Now we have the tools to ask what is required for the cell to perform each task. This has important implications not just for understanding how centromeric heterochromatin assembles, but also for learning how heterochromatin forms elsewhere on the chromosome, a process that is often disturbed in cancer.