A LOW COPY NUMBER CENTRAL SEQUENCE WITH STRICT SYMMETRY AND UNUSUAL CHROMATIN STRUCTURE IN FISSION YEAST CENTROMERE

Fission yeast centromeres vary in size but are organized in a similar fashion. Each consists of two distinct domains, namely, the approximately 15-kilobase (kb) central region (cnt + imr), containing chromosome-specific low copy number sequences, and 20- to 100-kb outer surrounding sequences (otr) with highly repetitive motifs common to all centromeres. The central region consists of an inner asymmetric sequence flanked by inverted repeats that exhibit strict identity with each other. Nucleotide changes in the left repeat are always accompanied with the same changes in the right. The chromatin structure of the central region is unusual. A nucleosomal nuclease digestion pattern formed on unstable plasmids but not on stable chromosome. DNase I hypersensitive sites correlate with the location of tRNA genes in the central region. Autonomously replicating sequences are also present in the central region. The behavior of truncated minichromosomes suggested that the central region is essential, but not sufficient, to confer transmission stability. A portion of the outer repetitive region is also required. A larger outer region is necessary to ensure correct meiotic behavior. Fluorescence in situ hybridization identified individual cens. In the interphase, they cluster near the nuclear periphery. The central sequence (cnt + imr) may play a role in positioning individual chromosomes within the nucleus, whereas the outer regions (otr) may interact with each other to form the higher-order complex structure.