We have investigated the chromatin structure of wild-type and mutationally altered centromere sequences in the yeast Saccharomyces cerevisiae by using an indirect endlabeling mapping strategy. Wild-type centromere DNA from chromosome III (CEN3) exhibits a nuclease-resistant chromatin structure 220-250 base pairs long, centered around the conserved centromere DNA element (CDE) III. A point mutation in CDE III that changes a central cytidine to a thymidine and completely disrupts centromere function has lost the chromatin conformation typically associated with the wild-type centromere. A second conserved DNA element, CDE I, is spatially separated from CDE HI by 78.86 A+T-rich base pairs, which is termed CDE H. The sequence and spatial requirements for CDE II are less stringent; alterations in CDE II length and sequence can be tolerated to a limited extent. Nuclease-resistant cores are altered in dimension in two CDE II CEN3 mutations. Two CDE I deletion mutations that retain partial centromere function also show nuclease-resistant regions of reduced size and intensity. The results from a number of such altered centromeres indicate a correlation between the presence of a protected core and centromere function.The centromere is a specific domain on eukaryotic chromosomes that plays an essential role in the faithful segregation of the chromosomes during mitotic and meiotic cell divisions. Cytologically, the centromere has been defined as the region of the chromosome that interacts with the spindle fibers during cell division. It has been proposed that centromerespecific DNA-binding proteins mediate the interaction between centromeres and spindle fibers. Following development of a functional assay for centromeres of the yeast Saccharomyces cerevisiae, the centromeres of 11 of the 16 chromosomes have been isolated (1-6). These centromere (CEN) sequences confer stable inheritance on autonomously replicating plasmids during mitosis and meiosis (1). Analysis and comparison ofthe DNA sequences of the 11 centromeres isolated has allowed description of several conserved sequence elements and their spatial organization (3,6,7). Each centromere has a highly conserved 8-base-pair (bp) In this report we have investigated the chromatin structure of several centromere mutations. We demonstrate that the nuclease-resistant chromatin core associated with the wildtype centromere is correlated with function. Specifically, point mutations in CDE III that destroy centromere function also destroy the nuclease-resistant region. The use of restriction enzymes as probes for chromatin structure confirms the change in structure associated with loss of centromere function. Centromere mutations that alter the distance between CDE I and CDE III alter the overall dimensions of the nuclease-resistant core. CEN mutations deleted for CDE I retain a residual nuclease-resistant structure that is reduced in intensity proportional to their loss of function.
MATERIALS AND METHODSStrains. S. cerevisiae strain J17 (MATa, his2, adel, trpl, metl4...