STRUCTURAL-ANALYSIS OF RDNA IN THE GENUS NICOTIANA

The nucleotide sequence of the intergenic spacer (IGS) region between the 25S and the 18S rRNA coding regions has been determined for tobacc o (Nicotiana tabacum). The IGS (5140 bp in length) can be subdivided i nto several regions (I-VII) two of which, upstream and downstream of t he putative transcription initiation site (TIS), contain prominent sub repeats (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 IG S sequences are compared with potato (Solanum tuberosum) and tomato (L ycopersicon esculentum) IGS. Restriction mapping of 13 Nicotiana speci es shows that considerable rDNA repeat length heterogeneity in this us is probably due to different numbers of A and C subrepeats.