Quantification of polymerases along transcribed rDNA genes. (A) Visualization of active genes in rDNA. Using a mutant strain with a reduced number of rDNA copies (strain NOY886; 42 rDNA copies), we obtained high quality spreads from total nucleolar DNA. (B) Quantification of actively transcribed rDNA genes. The quantification of active genes in mutant strain (42 active gene in NOY886) is compatible with previous results (French et al., 2003). (C) Visualization of individual polymerases on negatively stained Miller spread. In mutant strains with 25 rDNA copies, Pol I density on each gene is very high. Using higher magnification (bottom left), we can still detect individual polymerases, which can be quantified (right; red circles). Note that a nascent rRNA can be visualized from each detected polymerase.