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Molecular evolution of the intergenic spacer in the nuclear ribosomal RNA genes of Cucurbitaceae

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Summary

The intergenic spacer (IGS) of a 10-kbp repeat (clone pRZ7D) of the nuclear 18S, 5.8S, and 25S ribosomal RNA genes ofCucurbita pepo (zucchini) was sequenced and compared to the IGS sequences of two other Cucurbitaceae.Cucurbita maxima (squash), andCucumis sativus (cucumber). The nucleotide sequence and the structural organization of the IGS ofC. pepo andC. maxima are rather similar (between 75 and 100% sequence similarity depending on the region compared). The IGS are mainly composed of three different repeated elements interspersed into unique sequences: GC-rich clusters, a 422-bp AT-rich element including the transcription initiation site (TIS) for RNA polymerase I, and 260-bp repeats in the 5′ external transcribed spacer (D repeats). The TIS is duplicated in the 10-kbp repeat class ofC. pepo, as it is also described for the 11.5-kbp rDNA repeat ofC. maxima. The IGS ofCucumis sativus is also composed of different repeated elements; however, obvious sequence identity to theCucurbita species only occurs around the TIS and the preceding AT-rich region. GC-rich clusters with different primary sequences are present in the IGS of all three plants. Remarkably, the repeated elements in the 5′ETS accumulate TpG and TpNpG motifs, whereas CpG and CpNpG motifs less frequently occur. This accumulation might be caused by the transition of methylated cytosines (inmCpG ormCpNpG motifs) into thymidine via deamination in a previously GC-rich ancestor. The following singular region exhibits 50% G + C inC. pepo, 53% G + C inC. maxima, and 63% G + C inC. sativus. A model for a common ancestor of the 3′IGS for these Cucurbitaceae is proposed.

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Abbreviations

IGS:

(intergenic spacer)

ITS:

(internal transcribed spacer)

ETS:

(external transcribed spacer)

NTS:

(nontranscribed spacer)

rDNA:

(ribosomal DNA)

rRNA:

(ribosomal RNA)

TIS:

(transcription initiation site)

TTS:

(transcription termination site)

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Authors and Affiliations

  1. Institut für Mikrobiologie und Genetik der Universität München, Maria Ward Straße 1a, D-8000, München 19, Federal Republic of Germany

    Ramon A. Torres

  2. Biologisches Institut der Universität Tübingen, Lehrstuhl für Allgemeine Genetik, Auf der Morgenstelle 28, D-7400, Tübingen, Federal Republic of Germany

    Klaus King, Ulrike Zentgraf & Vera Hemleben

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  1. Klaus King
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  2. Ramon A. Torres
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  3. Ulrike Zentgraf
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  4. Vera Hemleben
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King, K., Torres, R.A., Zentgraf, U. et al. Molecular evolution of the intergenic spacer in the nuclear ribosomal RNA genes of Cucurbitaceae. J Mol Evol 36, 144–152 (1993). https://doi.org/10.1007/BF00166250

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  • DOI: https://doi.org/10.1007/BF00166250

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