A neomorphic sgs3 allele stabilizing miRNA cleavage products reveals that SGS3 acts as a homodimer

The putative RNA-binding protein SUPPRESSOR OF GENE SILENCING 3 (SGS3) protects RNA from degradation before transformation into dsRNA by the RNA-dependent RNA polymerase RDR6 during plant post-transcriptional gene silencing and trans-acting small interfering (siRNA) pathways. In this study, we show that SGS3 acts as a homodimer, and that the point mutation sgs3-3 impairs post-transcriptional gene silencing in a dominant-negative manner through the formation of SGS3/sgs3-3 heterodimers. Unlike complete-loss-of-function sgs3 mutants, which are impaired in the accumulation of both micro RNA-directed TAS cleavage products and mature trans-acting siRNAs, the sgs3-3 mutant overaccumulates TAS cleavage products and exhibits slightly reduced trans-acting siRNA accumulation. Together, these results suggest that sgs3-3 is a neomorphic allele that shows increased RNA protective activity, resulting in decreased RNA processing by downstream post-transcriptional gene silencing and trans-acting siRNA pathway components.