Grey boxes indicate the rDNA array and the black circles show the centromeres. Chromosome XII sequences surrounding the array are not drawn to scale. Positions of BamHI (B) and NgoMIV (N) restriction sites are indicated. BamHI has no sites within the rDNA or within the TRP1 insertion, whereas NgoMIV has no sites within the rDNA but a site within the TRP1 gene. Using CHEF gel analysis of BamHI- or NgoMIV-treated DNA samples, we determined the sizes of the rDNA clusters and the position of insertion of TRP1 within the clusters. We depict the location of the markers in the wild-type diploid AMC45. The sizes of the HPH-containing cluster, the positions of the TRP1 marker within the cluster (from the centromere-proximal junction), and the sizes of the URA3-containing clusters in kb for the other strains are, respectively: 1080, 880, 1110 (GAL-POL1 strain, AMC20); 1170, 940, 920 (fob1 strain, AMC156); 960, 580, 1110 (GAL-POL1 fob1 strain, AMC160).