COMMENTARY Higher-order chromosome structure in yeast

In this commentary I shall discuss recent findings on higher-order chromosome structure in yeast, a unicellular eukaryote. There are two kinds of yeast under current intensive investigation: namely, the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. I shall emphasize that yeast chromosomal proteins in many ways resemble those of higher eukaryotes. A major difference is the apparent absence of histone HI in yeast. Nevertheless, yeast chromosomal higherorder structure, and notably its manifestation in chromosome condensation, must have determinants in common with higher eukaryotes, including chromosomal proteins. I shall present examples of these, and some mutants relevant to them. They include DNA topoisomerases, protein kinases and phosphatases, and nuclear matrix (scaffold) proteins. In addition, I would like to stress that nuclear structure may be affected indirectly by proteins that maintain a proper nuclear environment.