Termination of transcription of ribosomal RNA genes of mung bean occurs within a 175 bp repetitive element of the spacer region

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Abstract
Summary In mung bean (Vigna radiata, formerly Phaseolus aureus) one length heterogeneity in the intergenic spacer (IGS) of the ribosomal DNA (rDNA) is due to a variable number of 175-bp subrepeats. This spacer region downstream of the 25S rRNA coding region was characterized by seuquencing the 2.4 kb EcoRI/HindIII fragment of a 10.5 kb mung bean rDNA repeat. Within the 175-bp repetitive elements a sequence was detected showing strong similarity to the T2/T3-box (GACTTGC) found in Xenopus rDNA and involved in termination and enhancing transcription. In mung bean this sequence partly forms the stem of a possible stem-loop structure at the 3′ end of each subrepeat. Nuclease mapping of transcription termination sites (TTS) results in two signals, 65 bp and 315 bp downstream of the 3′ end of the 25S rRNA coding region. The longer transcript terminates 20 bp downstream of the stem-loop structure at the end of the first 175-bp subrepeat. A spacer model is proposed which allows “readthrough enhancement”. No cross-hybridization was observed between the 180-bp subrepeats in pea (Pisum sativum) rDNA and the mung bean 175-bp subrepeat.