SEQUENCE HOMOLOGY REQUIREMENTS FOR TRANSCRIPTIONAL SILENCING OF 35S TRANSGENES AND POSTTRANSCRIPTIONAL SILENCING OF NITRITE REDUCTASE (TRANS)GENES BY THE TOBACCO-271 LOCUS

The transgene locus of the tobacco plant 271 (271 locus) is located on a telomere and consists of multiple copies of a plasmid carrying an N ptII marker gene driven by the cauliflower mosaic virus (CaMV) 19S pro moter and the leaf-specific nitrite reductase Nii1 cDNA cloned in the antisense orientation under the control of the CaMV 35S promoter. Prev ious analysis of gene expression in leaves has shown that this locus t riggers both post-transcriptional silencing of the host leaf-specific Nii genes and transcriptional silencing of transgenes driven by the 19 S or 35S promoter irrespective of their coding sequence and of their l ocation in the genome. In this paper we show that silencing of transge nes carrying Nii1 sequences occurs irrespective of the promoter drivin g their expression and of their location within the genome. This pheno menon occurs in roots as well as in leaves although root Nii genes sha re only 84% identity with leaf-specific Nii1 sequences carried by the 271 locus. Conversely, transgenes carrying the bean Nii gene (which sh ares 76% identity with the tobacco Nii1 gene) escape silencing by the 271 locus. We also show that transgenes driven by the figwort mosaic v irus 34S promoter (which shares 63% identity with the 35S promoter) al so escape silencing by the 271 locus. Taken together, these results in dicate that a high degree of sequence similarity is required between t he sequences of the silencing locus and of the target (trans)genes for both transcriptional and post-transcriptional silencing.