25S-18S rDNA intergenic spacer of Nicotiana sylvestris (Solanaceae): primary and secondary structure analysis

The complete nucleotide sequence (3701 bp) of the intergenic spacer region between the 3'-end of 25S rRNA gene and the 5'-end of 18S rRNA gene (IGS) i s described for Nicotiana sylvestris. The IGS of hr. sylvestris consists of seven structural regions (SR) two of which are composed of subrepeats (SR II: three and one half copies of the 76 bp D-subrepeat and numerous short C -subrepeats; SR IV: five copies of the 142/136 bp A-subrepeat). A presumpti ve rRNA processing signal (CTTTT) is located at the beginning of SR I. An i dentical sequence is also present in each copy of the A-subrepeat. Each D-s ubrepeat in SR IIa contains at the 5'-end the conserved sequence (GAGGTTTTT ) which may be involved in transcription termination. SR III (TG-rich) and SR IV (AT-rich) in the central part of IGS contain several potential replic ation origin sites. The transcription initiation site (TATATAAGGGGGG) is lo cated at the border of SR IV and V. No spacer promoters have been found. SR V contains four copies of a CATGG motif that is also present in SR II, VI and VII. At the 5'-end of SR VII there are three copies of the 66 bp highly divergent B-subrepeat. Presumptive pre-rRNA processing signals are also pr esent here. Possible hair-pin structures are described for SR V, VI and VII .