Large circular and linear rDNA plasmids in Candida albicans.

Although plasmids containing rRNA genes (rDNA) are commonly found in fungi, they have not been reported in Candida. We discovered that the yeast opportunistic pathogen Candida albicans contains two types of rDNA plasmids which differ in their structure and number of rDNA repeats. A large circular plasmid of unknown size consists of multiple rDNA repeats, each of which includes an associated autonomously replicating sequence (ARS). In contrast, a linear plasmid, which is represented by a series of molecules with a spread of sizes ranging from 50-150 kbp, carries a limited number of rDNA units and associated ARSs, as well as telomeres. The number of linear plasmids per cell is growth cycle-dependent, accumulating in abundance in actively growing cells. We suggest that the total copy number of rDNA is better controlled when a portion of copies are on a linear extrachromosomal plasmid, thus allowing a rapid shift in the number of corresponding genes and, as a result, better adaptation to the environment. This is the first report of a linear rDNA plasmid in yeast, as well as of the coexistence of circular and linear plasmids. In addition, this is a first report of naturally occurring plasmids in C. albicans.