Transcriptional silencing of a transgene by RNAi in the soma ofC. elegans

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Abstract
The silencing of transgene expression at the level of transcription in the soma ofCaenorhabditis elegansthrough an RNAi-dependent pathway has not been previously characterized. Most gene silencing due to RNAi inC. elegansoccurs at the post-transcriptional level. We observed transcriptional silencing when worms containing theelt-2::gfp/LacZtransgene were fed RNA produced from the commonly used L4440 vector. The transgene and the vector share plasmid backbone sequences. This transgene silencing depends on multiple RNAi pathway genes, includingdcr-1,rde-1,rde-4, andrrf-1. Unlike post-transcriptional gene silencing in worms,elt-2::gfp/LacZsilencing is dependent on the PAZ-PIWI protein Alg-1 and on the HP1 homolog Hpl-2. The latter is a chromatin silencing factor, and expression of the transgene is inhibited at the level of intron-containing precursor mRNA. This inhibition is accompanied by a decrease in the acetylation of histones associated with the transgene. This transcriptional silencing in the soma can be distinguished from transgene silencing in the germline by its inability to be transmitted across generations and its dependence on therde-1gene. We therefore define this type of silencing as RNAi-induced Transcriptional Gene Silencing (RNAi-TGS). Additional chromatin-modifying components affecting RNAi-TGS were identified in a candidate RNAi screen.

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