Figure 1.

Organization of rDNA and non-rDNA macronuclear chromosomes. (A) Schematic of the palindromic rDNA minichromosome. Expanded view of the 5′ NTS includes positioned nucleosomes (ovals) type I elements (black rectangles), pause site elements (grey rectangles), rRNA promoter (thin arrow) and replication origins (ori) which reside in the 430 bp imperfect duplicated sequences, domains 1 and 2 (thick arrows). The sequence of the type IB element T-rich strand and flanking DNA are shown, including predicted base pair interactions with 26T RNA. (B) Chromosome reorganization and replication during macronuclear development. A portion of the micronuclear chromosome encoding the single copy rRNA gene is shown (CBS: chromosome-breakage sequences). See text for additional details.