Structural studies on centromeres in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, circular or linear plasmids containing a functional centromere (CEN) and a chromosomal replicator (ARS) are mitotically stable and segregate as ordinary yeast chromosomes in the first and second meiotic divisions. A centromere in S. cerevisiae consists of a region of DNA, approximately 150 bp in length, containing three important sequence elements, which are folded with proteins into a specific conformation in the chromatin (the yeast kinetochore). Each of the functional CEN sequences contains a high (91% to 95%) AT region (element II), 78 to 86 bp in length, flanked on one side by the common sequence PuTCACPuTG (element I), and on the other by the sequence TGTTT.TG.TTTCCGAAA....AAA (element III). Deletions in the element II region partially inactivate mitotic function and cause precocious separation of the sister chromatids in meiosis I. Element III appears to be a protein binding site, as evidenced by the following observations. Various point mutations in element III inactivate centromere function, especially in the central CCG (17). One or more protein binding sites in the element III region can be demonstrated by an exonuclease III blocking assay. Wild-type CEN sequences compete strongly in this binding assay, whereas certain functionally inactive mutant CEN sequences do not. In addition, various DNA segments containing either CEN3 or the element III region strongly repress expression of the yeast GAL1 gene when inserted immediately upstream from the transcriptional start site. Helical DNA segments containing CEN3 or CEN14 are shown to be bent or distorted in shape in the high-AT element II region.