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6 Circular DNA Plasmids of Yeasts

James R. Broach, Fredric C. Volkert


In addition to chromosomes and mitochondrial DNA, the genomes of most yeasts encompass a variety of stable extrachromosomal elements. These include double-stranded DNA plasmids, such as 2-micron circle and 2-micron-circle-like plasmids described in this chapter; double-stranded RNA viruses, described by Wickner (this volume); and a variety of retrotransposons, reviewed by Boeke and Sandmeyer (this volume). Because these elements appear neither to confer any selective advantages to the cells in which they are resident nor to inflict any particular harm, they most likely represent various forms of benign molecular parasitism. As such, each type of extrachromosomal element has evolved various strategies to subvert aspects of the host’s reproductive machinery to its own ends without placing undue stress on the host itself.

Recent attempts to appreciate the molecular basis for the delicate tightrope act underlying the successful persistence of 2-micron circle have revealed novel and elegant processes of genome replication and segregation. Probing these processes has provided new and valuable perspectives on the analysis of DNA metabolism, cellular and nuclear architecture, and segregation of nuclear components during division. Not only have these studies uncovered unexpected mechanisms for selective amplification of specific DNA sequences and control of DNA replication, but they also have afforded the development of specialized vectors that have facilitated the use of yeast in the scientific and commercial application of genetic engineering.

In this paper, we summarize our current understanding of the molecular biology of 2-micron circle, with special emphasis on those processes responsible for its successful persistence.

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