EPIGENETIC REPEAT-INDUCED GENE SILENCING (RIGS) IN ARABIDOPSIS

In several plant systems expression of structurally intact genes may b e silenced epigenetically when a transgenic construct increases the co py number of DNA sequences. Here we report epigenetic silencing in Ara bidopsis lines containing transgenic inserts of defined genetic struct ure, all at the same genomic locus. These comprise an allelic series t hat includes a single copy of the primary insert, which carries repeat ed drug resistance transgenes, and a set of its derivatives, which as a result of recombination within the insert carry different numbers an d alleles of resistance genes. Although the drug resistance genes rema ined intact, both the primary and some recombinant lines nevertheless segregated many progeny that were partly or fully drug-sensitive becau se of silencing. As in other systems silencing was reversible, and cor related with decreased steady-state mRNA and increased DNA methylation . Each different number and combination of genes, on the same or diffe rent (i.e., homologous) chromosomes, conditioned its own idiosyncratic segregation pattern. Strikingly, lines with a single gene segregated only a few slightly drug-sensitive progeny whereas multi-gene lines se gregated many highly sensitive progeny, indicating dependence of silen cing at this locus on repeated sequences. This argues strongly against explanations based on antisense RNA, but is consistent with explanati ons based on ectopic DNA pairing. One possibility is that silencing re flects the interaction of paired homologous DNA with flanking heterolo gous DNA, which induces condensation of chromatin into a non-transcrib able state.