Figure 1

The rDNA locus and its R2 and R1 element insertions. (A) The rDNA locus is composed of a tandem array of rDNA units with a subset of these units inserted by R2 (blue boxes) and/or R1 elements (orange boxes). The rRNA transcription unit with external transcribed spacer (ETS), 18S, 5.8S and 28S genes (gray boxes), transcribed spacers (white boxes), and R2 insertion is diagrammed. The single open reading frame (ORF) of R2 is delineated in light blue. R2 RNA sequences are processed from the cotranscript at the 5' end by an R2 encoded self-cleaving ribozyme. After translation, identical subunits of the R2 protein (circles) bind sequences at either end of the R2 transcript, and the RNA/protein complex binds at the R2 target site in the 28S gene and proceeds with the insertion of a new R2 copy. (B) Diagram of a portion of the 28S gene with both R2 and R1 insertions. Arrows indicate location and direction of primers in the 28S gene and R2 element used to survey for unusual insertions near the R2 target site.