The organization and dynamics of a yeast chromosome arm

The organization of eukaryotic chromosomes in the interphase nucleus is non random. This organization stems in part from tethering to the nuclear membrane and is thought to play a role in gene regulation. To study the organization and dynamics of the left arm of chromosome III of Saccharomyces cerevisiae in vivo, we have placed fluorescent markers near the HML locus on chromosome III and at the spindle pole body (SPB), which serves as a marker of centromere attachment. We measured the end-to-end distances between these two loci in populations of fixed cells and found find that the removal of Yku80 and Esc1, two proteins involved in anchoring the telomere to the nuclear membrane, yields a distance distribution significantly different from that of the wild type. This suggests that telomere anchoring has an effect on the position of HML and its distance from the SPB. We quantitatively analyzed the distance histograms using a model of the chromosome as a random-walk polymer. We found that a simple Rabl model of organization is not sufficient to explain the experimental distributions but that further constraints on the model are needed in order for theoretical calculations to agree with the experimental data. To study the dynamics of the left arm we made use of the Optical Microscope-X (OMX) system which allowed us to take live cell images for long periods of time. We measured the mean square displacement between HML and the SPB and found that removing the telomere anchor led to a higher MSD value at long times suggesting that the motion of HML is increased when the anchor is removed. This is in contrast to our fixed cell data where the variance of the end to end distance distribution decreases in a yku80Delta esc1Delta mutant. When we further examine the diffusive motion between the two labeled loci, we see that it follows a subdiffusive law which is a characteristic of polymer diffusion.