Figure 15.3–The organization of higher plant ribosomal DNA cistron. NTS = nontranscribed spacer, ETS = external transcribed spacer, 18S = small-subunit rDNA, ITS-1 = first internal transcribed spacer, 5.8S = rDNA, ITS-2 = second internal transcribed spacer, 26S = large-subunit rDNA, TTS = transcription termination site. The rDNA cistron is tandemly repeated many times within the plant genome.