SummaryChromosomal rearrangements are common in both clinical isolates and spontaneous mutants of Candida albicans . It appears that many of these rearrangements are caused by translocations around the major sequence repeat (MSR) that is present in all chromosomes except chromosome 3, suggesting that homologous recombination (HR) may play an important role in the survival of this organism. In order to gain information on these processes, we have cloned the homologue of RAD52 , which in Saccharomyces cerevisiae is the only gene required for all HR events.

Rap1 (repressor activator protein 1) is a conserved multifunctional protein initially identified as a transcriptional regulator of ribosomal protein genes in Saccharomyces cerevisiae but subsequently shown to play diverse functions at multiple chromosomal loci, including telomeres. The function of Rap1 appears to be evolutionarily plastic, especially in the budding yeast lineages. We report here our biochemical and molecular genetic characterizations of Candida albicans Rap1, which exhibits an unusual, miniaturized domain organization in comparison to the S. cerevisiae homologue. We show that in contrast to S. cerevisiae, C. albicans RAP1 is not essential for cell viability but is critical for maintaining normal telomere length and structure. The rap1 null mutant exhibits drastic telomere-length dysregulation and accumulates high levels of telomere circles, which can be largely attributed to aberrant recombination activities at telomeres. Analysis of combination mutants indicates that Rap1 and other telomere proteins mediate overlapping but nonredundant roles in telomere protection. Consistent with the telomere phenotypes of the mutant, C. albicans Rap1 is localized to telomeres in vivo and recognizes the unusual telomere repeat unit with high affinity and sequence specificity in vitro. The DNA-binding Myb domain of C. albicans Rap1 is sufficient to suppress most of the telomere aberrations observed in the null mutant. Notably, we were unable to detect specific binding of C. albicans Rap1 to gene promoters in vivo or in vitro, suggesting that its functions are more circumscribed in this organism. Our findings provide insights on the evolution and mechanistic plasticity of a widely conserved and functionally critical telomere component.Multifunctional Rap1 (repressor activator protein 1) was first discovered in the budding yeast Saccharomyces cerevisiae as a positive transcriptional regulator of multiple growth-related genes such as the ribosomal protein genes (23). Other studies identified Rap1 as the major double-strand telomere repeat binding protein in S. cerevisiae and to be essential for maintaining telomere length and structural integrity (5, 10). Upon further analysis, Rap1 was recognized as a key component of the mating-type silencer and shown to be required for transcriptional silencing (7,43,44). These disparate observations raised fascinating questions concerning the mechanisms whereby a single protein participates in diverse functions at distinct chromosomal locations. The identification of Rap1 homologues in humans and the fission yeast resulted in yet more surprises (25,29). Both homologues were shown to be telomere-associated proteins required for proper telomere length regulation. However, instead of binding DNA directly, human and Schizosaccharomyces pombe Rap1 proteins were recruited to telomeres through interaction with other telomere proteins such as TRF2 and Taz1. Moreover, there is no evidence that the human and S. pombe proteins are involved in transcriptional regulation. Thus, although th...