Prp20, the Saccharomyces cerevisiae homolog to the mammalian regulator of chromosome condensation, RCC1, binds to double-stranded (ds) DNA in vitro through a multi-component complex. Three members of this complex bind GTP in vitro. The Prp20 complex specifically loses its ability to bind dsDNA during DNA replication as determined by an in vitro assay using cell extracts from arrested cdc mutants. This loss of dsDNA-binding activity does not affect the proper organization of the nucleoplasm as was the case for the prp20-1 and prp20-7 mutants, suggesting a segregation exists in the biochemical activities of the Prp20 protein. Detailed analysis of Prp20 demonstrates that specific and highly conserved amino acids coordinate these distinct activities of the Prp20 complex. These essential residues are mainly located in the second and the third repeats of the amino terminus and the last two repeats of the carboxyl terminus of Prp20. Furthermore, mutations in the last two repeats are suppressed by Gsp1, one of the GTP-binding components of the Prp20 complex, but not the mutations in the amino-terminus of Prp20.

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