Figure 1.

Schematic of the rDNA gene organization in Saccharomyces cerevisiae. The rDNA array encodes 100–200 repeats of the 5S and 35S rDNA genes separated by non-transcribed spacers (NTS). NTS2 contains the ARS (autonomous replicating sequence) and the I element of HOT1, which consists of the promoter and the transcriptional start. The end of the 35S pre-RNA contains the E element of the HOT1 sequence, which overlaps the 35S enhancer. The thick bar marks the transcribed spacer sequence. The 5΄ and 3΄ ends of the E and I elements are noted with the 5΄ end of the 35S pre-rRNA. The direction of transcription is depicted with an arrow above each gene. The 5΄ and 3΄ external transcribed spacers (ETS) are indicated.