NATIONAL INSTITUTE FOR BASIC BIOLOGY
Figure 2. Physical map of the rRNA gene repeat and location of the replication fork block (SOG) site. The direction of transcription of the 35S and 5S rRNA genes (open arrow), the mature 18S, 5.8S, 26S and 5S rRNAs (closed bar), the introns (open bar), the two non-transcribed spacer regions (NTS1 and NTS2), ori (replication origin) and I-element (a component of HOT1 DNA) are indicated. BglIIA and B DNA fragments are produced when rDNA is digested with BglII restriction enzyme. NTS1 and its surrounding region are expanded (Bell et al., 1977), under which three solid bars represent the E-element (another HOT1 component), enhancer (enhancer element of 35S rRNA transcription) and SOG (replication fork blocking site; in this case the SOG can inhibit the fork traveling from the right side only). The relevant restriction sites are also shown.