Nowadays it seems like “interference” carries such negative connotations. RNAs aren’t really big meddlers. In fact, it’s RNAi that’s protecting genomes against the transgenerational loss of DNA methylation. A team of researchers in France and Spain has demonstrated that in Arabidopsis, the RNA interference (RNAi) machinery can remethylate target sequences that were hypomethylated by mutation of the chromatin remodeler gene DDM1. Their results are reported in the March 20, 2009, issue of Science.
Mutation of DDM1 results in the hypomethylation and reactivation of harmful transposable elements in the genome. Previous transgenerational evidence in plants has suggested that once DNA methylation is compromised by DDM1 mutation, it cannot be restored in subsequent generations by replacement of the wild-type allele. However, by analyzing the methylation status of 56 transposable and other repeat elements in the DDM1/DDM1 F5 progeny of ddm1 mutants, the investigators discovered that some of the repeat elements had wild-type methylation levels and had been resilenced.
Analysis of deep sequencing data revealed that, in contrast to the “unremethylatable” repeat elements, these “remethylatable” sequences had an abundance of corresponding 24-nt siRNAs. These results suggest that over several generations, the RNAi-dependent de novo methylation machinery can drive the progressive remethylation of some hypomethylated repeat sequences, which could protect genomes against transgenerational epigenetic defects. Get all the details at Science, March 2009.