Bdf1, a yeast chromosomal protein required for sporulation.

The BDF1 gene of Saccharomyces cerevisiae is required for sporulation. Under starvation conditions, most cells from thebdf1null mutant fail to undergo one or both meiotic divisions, and there is an absolute defect in spore formation. The Bdf1 protein localizes to the nucleus throughout all stages of the mitotic and meiotic cell cycles. Analysis of spread meiotic nuclei reveals that the Bdf1 protein is localized fairly uniformly along chromosomes, except that it is excluded specifically from the nucleolus. Abdf1null mutant displays a reduced rate of vegetative growth and sensitivity to a DNA-damaging agent. TheBDF1gene encodes a 77-kDa protein that contains two bromodomains, sequence motifs of unknown function. Separation-of-function alleles suggest thatonlyoneofthetwobromodomainsisrequiredforsporulation,whereasbotharerequiredforBdf1function in vegetative cells. We propose that the Bdf1 protein is a component of chromatin and that the mitotic and meiotic defects of thebdf1null mutant result from alterations in chromatin structure. Diploid eukaryotic organisms can undergo two different types of cell division, mitosis and meiosis. During mitosis and the second division of meiosis, sister chromatids separate and segregate, while homologous chromosomes behave independently. In contrast, during thefirst division of meiosis, homologous chromosomes pair and recombine with each other and then segregate to opposite poles. Although many of the proteins required for a successful meiotic cell cycle are synthesized specifically during meiosis, proteins present in both vegetative and meiotic cells play important roles as well (65). Some proteins serve identical functions during the two types of cell division, but others act differentially. For example, several CDC gene products of Saccharomyces cerevisiae are required for the G1-to-S transition in the mitotic cell cycle, but some of these act after DNA replication during meiosis (64). Still other gene products serve functions of different importance in the two types of cell cycle. For instance, genes in the yeastRAD50series are required for recombinational repair of DNA damage but are otherwise dispensable for vegetative growth (17). However, these genes are essential for the generation of viable meiotic products, because recombination is required for reductional chromosome segregation (53).