Plant Molecular Biology 35: 655–660, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Structural analysis of rDNA in the genus Nicotiana
Nikolai V. Borisjuk
, Yurij M. Davidjuk
, Stepan S. Kostishin
, Galina P. Miroshnichenco
, Vera Hemleben
and Roman A. Volkov
ur Pﬂanzengenetik und Kulturpﬂanzenforschung, Corrensstrasse 3, D-06466 Gatersleben, Germany
author for correspondence);
Department of Biochemistry, University of Chernivtsy, Kotsubinsky Street 2,
274012 Chernivtsy, Ukraine;
Institute of Physical and Chemical Biology, University of Moscow, 119899 Moscow,
ur Allgemeine Genetik, Biologisches Institut, Universit
Received 23 December 1996; accepted in revised form 18 June 1997
Key words: ribosomal DNA, intergenic spacer, tobacco, molecular evolution, Lycopersicon, Solanum, Solanaceae.
The nucleotide sequence of the intergenic spacer (IGS) region between the 25S and the 18S rRNA coding regions
has been determined for tobacco (Nicotiana tabacum). The IGS (5140 bp in length) can be subdivided into several
regions (I–VII) two of which, upstream and downstream of the putative transcription initiation site (TIS), contain
prominent subrepeats (A and C). The unique sequence in the central part of the IGS (region IV) preceding the
TIS is extremely AT-rich. The distance from the putative TIS to the 5
end of the 18S rRNA gene is 3005 bp.
The IGS sequences are compared with potato (Solanum tuberosum) and tomato (Lycopersicon esculentum)IGS.
Restriction mapping of 13 Nicotiana species shows that considerable rDNA repeat length heterogeneity in this
genus is probably due to different numbers of A and C subrepeats.
The 18S and 25S ribosomal RNA genes (rDNA) serve
as a convenient model for molecular studies of evol-
utionary relationships and for the investigations of
molecular changes in multigene families. In euka-
ryotes, rDNA is located at the nucleolar organiser
region (NOR) and consists of tandemly repeated units
composed of transcribed regions coding for the 18S,
5.8S and 25–28S rRNA and of several spacer regions
The genus Nicotiana (Solanaceae) is a suitable
model system for the investigation of plant speciation
andgenome evolution [7, 13]. Thereexist 60–95Nico-
tiana species in America, Australia and Africa [4, 7].
Despite the great interest in Nicotiana, information on
rDNA organization in this genus is still limited [3, 18,
We have sequenced the intergenic spacer region
(IGS) of N. tabacum rDNA and compared it with the
GenBank and DDBJ Nucleotide Sequence Databases under the
accession number Y08422.
IGS of S. tuberosum  and L. esculentum . In
order to describe rDNA organisation in other repres-
entatives of the genus Nicotiana comparative restric-
tion mapping was performed for 13 South Americ-
an species which have different chromosomal num-
bers and which belong to 3 subgenera : Tabacum
(sect. Tomentosae: N. otophora, N. tomentosiformis;
sect. Genuine: N. tabacum), Petunioides (sect. Alatae:
N. alata, N. langsdorfﬁi, N. longiﬂora, N. plumbagini-
folia, N. sanderae, N. sylvestris), and Rustica (sect.
Paniculatae: N. glauca, N. knigtiana, N. paniculata,
rDNA mapping, cloning and sequencing
rDNA restrictionmappingwas performedas described
recently [2, 3, 18]. N. tabacum DNA was digested
formedinto Escherichiacolistrain ‘Sure’(Stratagene).
The restriction mapfor clone pNt-4(accession number
Y08422) containing the complete IGS of N. tabacum
Gr.: 201001135, PIPS Nr. 144477 BIO2KAP
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