Characterization of Schizosaccharomyces pombe minichromosome deletion derivatives and a functional allocation of their centromere

Distribution of bent DNA structures in the fission yeast centromere

To gain a clue as to the functional significance of DNA curvature, we experimentally characterized the distribution of bent DNA structures throughout the 35-kb cen1 sequence, one of the isolated functional centromeric DNA of the fission yeast, Schizosaccharomyces pombe. It was revealed that a relatively large central portion of cen1, covering a 2.2-kb DNA sequence, displays a remarkable DNA curvature.

Somatic instability of a Drosophila chromosome

A mitotically unstable chromosome, detectable because of mosaic expression of marker genes, was generated by X-ray mutagenesis in Drosophila. Nondisjunction of this chromosome is evident in mitotic chromosome preparations, and premature sister chromatid separation is frequent. The mosaic phenotype is modified by genetic elements that are thought to alter chromatin structure. We hypothesize that the mitotic defects result from a breakpoint deep in the pericentric heterochromatin, within or very near to the DNA sequences essential for centromere function. This unique chromosome may provide a tool for the genetic and molecular dissection of a higher eukaryotic centromere.

Structure of the fission yeast centromere cen3: direct analysis of the reiterated inverted region

We determined the structure of the Schizosaccharomyces pombe centromere cen3 using direct genomic mapping and cosmid walking. The repetitive region of cen3 is approximately 110 kb, much longer than that of the previously determined cen1 and cen2 regions. The approximately 30 kb long left and approximately 60 kb right repetitive sequences are arranged with an inverted symmetry and flank the 15 approximately 20 kb central domain. The repeat motifs in cen3, although they consist of the common centromeric repeat elements, are slightly different from those in cen1 and cen2. The cen3 repeat motifs appear to be reiterated four times in the left and nine times in the right side repetitive regions. We found that the central domain consists of the common approximately 5 kb core sequence associated with the pair of innermost inverted sequences, most of which are reiterated only twice in the genome. Although their sizes differ significantly, the general features of cen1, cen2 and cen3 are similar, and a prototype, consensus structure for the fission yeast centromere may be deduced.

A large number of tRNA genes are symmetrically located in fission yeast centromeres

We report here that the fission yeast centromere regions in the three chromosomes contain no less than 36 symmetrically arranged tRNA-coding sequences, and many of them are located within the inner inverted regions that are thought to be essential for the centromere function. There are 11 different species of tRNA-coding sequences, and four of them are identical to those previously known in this organism. This high-density distribution of tRNA genes in the centromere regions is surprising, as the fission yeast centromeres were thought to form transcriptionally inactive structures.

The fission yeast cut1+ gene regulates spindle pole body duplication and has homology to the budding yeast ESP1 gene

Mutations in the fission yeast cut1+, cut2+, and cut10+ genes uncouple normally coordinated mitotic events and deregulate, rather than arrest, mitosis. DNA synthesis continues, making polyploid nuclei with several spindles. Multiple, aberrant spindle pole bodies (SPBs) are produced in cut1 mutant cells. The cut1+ and cut2+ genes are cloned by transformation. High gene dosage of cut1+ also complements cut2 and cut10 mutants. The cut2+ gene, however, complements only cut2. The 210 kd cut1+ gene product contains putative ATP binding and helical coil regions followed by a COOH-terminal domain homologous to the S. cerevisiae gene ESP1. Mutations in the ESP1 gene also result in many SPBs. The cut1+ product is shown by anti-cut1 antibody to be a rare component of the insoluble nuclear fraction. It may play a key role in coupling chromosome disjunction with other cell cycle events and is potentially a component, regulator, or motor for the SPB and/or kinetochores.

Centromeres of budding and fission yeasts

Centromeres of the budding yeast Saccharomyces cerevisiae are structurally relatively simple, are specified by only about 125 base pairs of DNA, and contain no repeated DNA sequences. The centromere regions in the fission yeast Schizosaccharomyces pombe span many kilobase pairs of DNA and contain repeated DNA sequences that appear to be necessary for full centromere function. A portion of the repeated sequences is organized into a large inverted repeated structure in the centromere region of each S. pombe chromosome. Fission yeast provides an excellent model system for studying the role of repeated DNA sequences in centromere function.