Abstract
The ability of a functional gene to complement a nonfunctional gene may depend upon the intracellular relationship of the two genes. If so, the function of the gene product in question must be limited in time or in space. CDC (cell division cycle) gene products of Saccharomyces cerevisiae control discrete steps in cell division; therefore, they constitute reasonable candidates for genes that function with temporal or spatial restrictions. In an attempt to reveal such restrictions, we compared the ability of a CDC gene to complement a temperature-sensitive cdc gene in diploids where the genes are located within the same nucleus to complementation in heterokaryons where the genes are located in different nuclei. In CDC X cdc matings, complementation was monitored in rare heterokaryons by assaying the production of cdc haploid progeny (cytoductants) at the restrictive temperature. The production of cdc cytoductants indicates that the cdc nucleus was able to complete cell division at the restrictive temperature and implies that the CDC gene product was provided by the other nucleus or by cytoplasm in the heterokaryon. Cytoductants from cdc28 or cdc37 crosses were not efficiently produced, suggesting that these two genes are restricted spatially or temporally in their function. We found that of the cdc mutants tested 33 were complemented; cdc cytoductants were recovered at least as frequently as CDC cytoductants. A particularly interesting example was provided by the CDC4 gene. Mutations in CDC4 were found previously to produce a defect in both cell division and karyogamy. Surprisingly, the cell division defect of cdc4 nuclei is complemented by CDC4 nuclei in a heterokaryon, whereas the karyogamy defect is not.