Rajeswaran, R. | bioRxiv Science

bioRxiv2026

RNAi-based discrimination of exogenous DNA by mitotic heterochromatin in fission yeast

Various mechanisms protect prokaryotic cells from invasive exogenous (exo)DNA. However, these are rarely conserved in eukaryotes, where the existence of such processes remains elusive. Here, we uncover how fission yeast eliminates exoDNA through unequal partitioning. We show that intrinsic transcriptional features of plasmidial exoDNA produce small interfering (si)RNAs that recruit heterochromatin onto the plasmid. An active, heterochromatin-dependent clustering mechanism then causes its asymmetric partitioning during mitosis. The mitotic kinase Aurora B, which dissociates heterochromatin on segregating chromosomes, is distinctively hypoactive on plasmidial DNA, facilitating its unequal partitioning and, ultimately, elimination from the cell population. Thus, an interplay between RNAi-mediated exoDNA heterochromatinization and chromosomal phosphorylation enables fission yeast to discriminate self-from non-self-DNA, uncovering a hitherto undescribed cell-autonomous immune process in eukaryotic cells.

cell biology↗

bioRxiv2021

Innate, translation-dependent silencing of an invasive transposon in Arabidopsis

Co-evolution between hosts and parasites genomes shapes diverse pathways of acquired immunity based on silencing small (s)RNAs. In plants, sRNAs cause heterochromatinization, sequence-degeneration and, ultimately, loss-of-autonomy of most transposable elements (TEs). Recognition of newly-invasive plant TEs, by contrast, involves an innate antiviral-like silencing response. To investigate this responses activation, we studied the single-copy element EVADE (EVD), one of few representatives of the large Ty1/Copia family able to proliferate in Arabidopsis when epigenetically-reactivated. In Ty1/Copia-elements, a short subgenomic mRNA (shGAG) provides the necessary excess of structural GAG protein over the catalytic components encoded by the full-length genomic flGAG-POL. We show here that the predominant cytosolic distribution of shGAG strongly favors its translation over mostly-nuclear flGAG-POL, during which an unusually intense ribosomal stalling event coincides precisely with the starting-point of sRNA production exclusively on shGAG. mRNA breakage occurring at this starting-point yields unconventional 5OH RNA fragments that evade RNA-quality-control and concomitantly likely stimulate RNA-DEPENDENT-RNA-POLYMERASE-6 (RDR6) to initiate sRNA production. This hitherto-unrecognized "translation-dependent silencing" (TdS) is independent of codon-usage or GC-content and is not observed on TE remnants populating the Arabidopsis genome, consistent with their poor association, if any, with polysomes. We propose that TdS forms a primal defense against de novo invasive TEs that underlies their associated sRNA patterns.

plant biology↗