PHYSICAL CONSTITUTION OF RIBOSOMAL GENES IN COMMON STRAINS OF SACCHAROMYCES-CEREVISIAE

Several recent investigations, employing restriction endonucleases that do not cleave within rDNA units, revealed that a number of laboratory strains of Saccharomyces cerevisiae apparently contain a single tandem array of approximately 50 to 200 rDNA units on each chromosome XII homolog. The number of these rDNA units varies from strain to strain, among subclones of the same strain, and after different conditions of growth. In contrast, the commonly-used strain S288C and its derivatives contain two clusters on each chromosome XII homolog. Although the two clusters are stably maintained, the number of rDNA units within each cluster can vary as in strains with single clusters.