Required for the heritable maintenance of transcriptional gene silencing (TGS) in heterochromatin, and particularly in the intermediate bivalent heterochromatin, characterized by an unsual methylation pattern consisting in hypermethylated DNA and histone H3 'Lys-4' methylation (H3K4me) together with depletion of histone H3 'Lys-9' methylation (H3K9me), in a chromatin methylation-independent manner, in a non-CG methylation context. May play a role in the RNA polymerase IV/V (Pol-IV/V)-mediated RNA-directed DNA methylation (RdDM) leading to TGS (also called siRNA-mediated TGS pathway), probably by modulating small interfering RNA (siRNA) accumulation. Especially involved in the gene silencing of the transcriptionally silent information region (TSI), 5S ribosomal RNA genes (localized in the pericentromeric heterochromatin of chromosomes 3, 4, and 5) and of 180-bp satellite repeats and 106B long terminal repeat (LTR)-like repeats of the chromocenters. Prevents the aberrant mRNA transcriptional read-through. Reactivated transcription of heavily methylated silent loci, independently of chromatin demethylation. Releases TSI, 5S ribosomal RNA genes as well as 180-bp and 106B repeats of chromocenters silencing independently of chromatin properties. Partial derepression of 35S promoter homology-dependent TGS in transgenic plants. Abnormal aberrant mRNA transcriptional read-through. Increased expression of minor 5S rRNA species. No nuclear organization alteration. Requires for the complete maintenance of the 35S promoter homology-dependent TGS induced by T-DNA construct insertion. Truncated N-terminus.