Complex organization of the length heterogeneous 5′ external spacer of mung bean (Vigna radiata) ribosomal DNA

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Abstract
Restriction enzyme analysis and cloning of the 18S, 5.8S, and 25S ribosomal RNA genes (rDNA) of the mung bean (Vigna radiata = Phaseolus aureus) reveal length heterogeneity in the repeating units (10 – 11 kbp) localized within two different regions in the ribosomal spacer. The 1.5 – 2.0 kbp region flanking the 3′ end of the 25S rRNA contains various numbers (8 – 10) of tandemly arranged 180 bp subrepeats. After DNA sequencing a complex organized length heterogeneous 5′ external spacer built up of different numbers of 340 bp subrepeats, each flanked by 52 bp direct repeats, is detected and described for the first time for plant ribosomal DNA repeating units. Sequences occurring in front of and within this repeated structure (elements II – IV) can be combined with the motifs P1, P2, and P3. These exhibit a strong similarity to transcription initiation sites specific for RNA polymerase I described for other plant and animal rDNA investigated to date. Transcription products complementary to the complex repeated structures are detected by hybridization to total nuclear RNA. The 9 bp element V located in front of the first 340 bp region appears in duplicated form as a direct repeat with sequence similarity to SV40 (or RNA polymerase II) enhancer sequences.Key words: promoter, ribosomal RNA genes, RNA polymerase I, spacer organization, transcription.