Heat-shock induced accumulation of extremely long transcripts containing the rDNA intergenic spacer in arabidopsis

A cDNA clone containing a 1.2-kb fragment derived from the ribosomal DNA (rDNA) intergenic spacer (IGS) of Arabidopsis was characterized. Isolation of this clone was achieved in a previous study by differential screening of a cDNA library prepared from heat-shocked Arabidopsis seedlings (Takahashi and Komeda 1989). Nucleotide sequence analysis revealed that the cDNA had an oligo(dA) sequence that corresponded to the sequence found in the IGS. Northern blot analysis using the cloned cDNA as probe demonstrated that heat-shock treatment of seedlings induced the accumulation of extremely long transcripts with apparent lengths of more than 7 kb. Treatment with arsenite and cadmium ions also led to the accumulation of the RNA, a phenomenon very similar to that observed in the case of expression of genes for heat-shock proteins (HSPs). Although the possibility remains to be excluded that these transcripts might be derived from a region homologous to the IGS located outside the rDNA clusters in Arabidopsis, it is most likely that processing of precursor rRNA and/or termination of transcription of rRNA are greatly impaired under stressed conditions in plants, as reported in the case of animal genes for rRNA (Labhart and Reeder 1987, Parker and Bond 1989).