Abstract
Decrease of RNAi efficiency during culture is a common phenomenon in wide range of species. We showed this is due to the epigenetic transcriptional repression for the silencer construct, and Elongin C, which is a component of ubiquitin ligase, is enrolled in this reaction.
Frequency of methyl-CpG was high in the inverted repeat region in Chlamydomonas of the low RNAi efficient cells. Moreover, in these cells, histone modifications in the inverted repeat and promoter regions were repressive types. These show decrease of RNAi efficacy is due to epigenetic silencing for the silencer construct. We obtained a tag insertional mutant that recovered the RNAi efficiency. Elongin C was deleted in this tag-mutant. In this mutant, histone modifications in the promoter region were changed to active ones, while histone modifications and the frequency of methyl-CpG in the inverted repeat were not changed. These show nucleation of heterochromatin formation started in the inverted region, which is initiated by accumulation of methyl-CpG, extended over the promoter region where methyl-CpG is limited, and Elongin C containing complex is essential to the extension of heterochromatin formation.
