Parsa, J.-Y. | bioRxiv Science

bioRxiv2018

Chromatin rewiring mediates programmed evolvability via aneuploidy

Eukaryotes have evolved elaborate mechanisms to ensure that chromosomes segregate with high fidelity during mitosis and meiosis1, and yet specific aneuploidies can be adaptive during environmental stress2,3. Here, we identify a chromatin-based system for inducible aneuploidy in a human pathogen. Candida albicans utilizes chromosome missegregation to acquire resistance to antifungal drugs4,5 and for ploidy reduction after mating6. We discovered that the ancestor of C. albicans and two related pathogens evolved a variant of histone H2A that lacks the conserved phosphorylation site for Bub1 kinase7, a key regulator of chromosome segregation1. Expression of this variant controls the rates of aneuploidy and antibiotic resistance in this species. Moreover, CENP-A/Cse4, the histone H3 that specifies centromeres, is depleted from tetraploid mating products and virtually eliminated from cells exposed to aneuploidy-promoting cues. Thus, changes in chromatin regulation can confer the capacity for rapid evolution in eukaryotes.

genetics↗

bioRxiv2017

Polymerase pausing induced by sequence-specific RNA binding protein drives heterochromatin assembly

Packaging of pericentromeric DNA into heterochromatin is crucial for genome stability, development and health, yet its endogenous triggers remain poorly understood1. A defining feature of pericentromeric heterochromatin is histone H3 lysine 9 methylation (H3K9me)2-4. In S. pombe, transcripts derived from the pericentromeric dg and dh repeat during S phase5-7 promote heterochromatin formation through two pathways: an RNAi-dependent mechanism involving recruitment of the Clr4 H3K9 methyltransferase complex (CLR-C) via the RITS complex8-13, and RNAi-independent mechanism involving an RNAPII-associated RNA-binding protein Seb1, the repressor complex SHREC, and RNA processing activities14-19. We show here that Seb1 promotes long-lived RNAPII pausing. Pause sites associated with sequence-specific Seb1 RNA binding events are significantly enriched in pericentromeric repeat regions and their presence correlates with the heterochromatin-triggering activities of the corresponding dg and dh DNA fragments. Remarkably, globally increasing RNAPII stalling by other means induces the formation of novel large ectopic heterochromatin domains. Such ectopic heterochromatin occurs even in cells lacking functional RITS, demonstrating that RNAPII pausing can be sufficient to trigger de novo heterochromatin independently of RNAi. These results uncover Seb1-mediated polymerase stalling as a new signal for nucleating heterochromatin assembly in repetitive DNA.

molecular biology↗