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Figure 1. Densities of engaged polymerases. A) The range of transcriptional activity of all ORFs and essential ORFs in bacteria and yeast. The number of ORFs with different polymerase densities is indicated for bacteria grown in LB or M9 + Glc and for yeast in YPD. B) The number of polymerases engaged on successive 50 kbp segments (each containing 
50 genes) of the E. coli genome. Positions of the (active) rrn operons (A–G) encoding rRNA and the origin (ori) plus terminus (ter) are indicated. Polymerases switch from the rrn operons to ORFs upon transfer from LB to M9 + Glc.
