Untitled

Abstract

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.

Molecular Organization of the 25S–18S rDNA IGS of Fagus sylvatica and Quercus suber: A Comparative Analysis

“…Exploring the IGS sequences for functional elements and domains we have predicted the putative transcription initiation site (TIS) in both spacers: TCTTT A GGGGGG (position - 5 relative to the initiating A) (Figure 5) through the alignment of our IGSs with the regions of initiation of transcription of other species from the Fagaceae [8], Cucurbitaceae [11], [42], [43], Fabaceae [44], [45], Brassicaceae [9], [13], [46]–[49], Solanaceae [3], [50], [51], as well as three monocots [52]–[54]. We have also found two adjacent CAAT-boxes, which is a common cis -acting element in promoter and enhancer regions (position −156 in Beech and −50 in Cork Oak spacer).…”

“…Fagus sylvatica and Quercus suber ribosomal intergenic spacers showed, like in the white oaks [8], an organization typical of most ribosomal IGS composed by five different regions: sub-repeats (SR), AT-rich, promoter and external transcribed sequences (3′ and 5′-ETS) - with structural features of plant IGS sequences and all functional elements needed for rRNA gene activity [3]–[5], [8], [11], [47], [49], [50], [58], [59].…”

“…The IGS located between the 3' end of the 25S rRNA gene and the 5' end of the 18S rRNA gene comprises the 3′ external spacer (3′-ETS), the non-transcribed region (NTS), and the 5' external transcribed spacer (5′-ETS) [2]. The rRNA genes are more conserved across species than the NTS region, which show sequence and length variation between species, populations, and even within individuals [3]–[7].…”

“…The presence of conserved structural features such as several types of repeating elements (or sub-repeats) functioning as enhancers, sequences with self-complementarity that could generate a conserved secondary structure, transcription initiation (TIS), and termination sites (TTS), which are involved in the regulation of transcription of the 18-5.8-25S rDNA cistrons led to the recognition of the NTS as a functional important region [2]–[4], [8]. Comparative studies have also been performed in several plant species of the same and different families [3], [8]–[10]. The 25S-18S IGS of almost all plant species studied so far show length heterogeneity mainly due to duplications or deletions of the sub-repeat region (SR) that can be present in different numbers and arranged in very complex patterns [8], [11], [12], of the 5′-ETS [3], [4], and duplications of the promoter [8], [13].…”

“…Comparative studies have also been performed in several plant species of the same and different families [3], [8]–[10]. The 25S-18S IGS of almost all plant species studied so far show length heterogeneity mainly due to duplications or deletions of the sub-repeat region (SR) that can be present in different numbers and arranged in very complex patterns [8], [11], [12], of the 5′-ETS [3], [4], and duplications of the promoter [8], [13].…”