Abstract. Topoisomerase II (topolD and RAP1 (Repressor Activator Protein 1) are two abundant nuclear proteins with proposed structural roles in the higherorder organization of chromosomes. Both proteins cofractionate as components of nuclear scaffolds from vegetatively growing yeast cells, and both proteins are present as components of pachytene chromosomes, cofractionating with an insoluble subfraction of meiotic nuclei. Immunolocalization using antibodies specific for topolI shows staining of an axial core of the yeast meiotic chromosome, extending the length of the synaptonemal complex. RAP1, on the other hand, is located at the ends of the paired bivalent chromosomes, consistent with its ability to bind telomeric sequences in vitro. In interphase nuclei, again in contrast to antitopolI, anti-RAP1 gives a distinctly punctate staining that is located primarily at the nuclear periphery. Approximately 16 brightly staining foci can be identified in a diploid nucleus stained with anti-RAP1 antibodies, suggesting that telomeres are grouped together, perhaps through interaction with the nuclear envelope.

We have used an immunoprecipitation‐PCR cycle to isolate physically genomic DNA sequences that are bound by the fission yeast cdc10 gene product in an attempt to identify novel target genes. An essential gene, cdt1, has been isolated whose expression is cell cycle regulated in a cdc10 dependent manner. The cdt1 promoter contains a recognition site for a sequence specific DNA binding factor. The cdc10 gene product is a component of this factor. Ectopic expression of cdt1 can complement a temperature sensitive mutation of cdc10 at semipermissive temperature. Cells carrying a null allele of cdt1 are defective in DNA replication but initiate mitotic events, suggesting that cdt1 is essential for the normal dependency relationship of S‐phase and mitosis.

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