Structural basis of HapEP88L-linked antifungal triazole resistance in Aspergillus fumigatus

Azoles are first-line therapeutics for human and plant fungal infections, but their broad use has promoted the development of resistances. Recently, a pan-azole–resistant clinical Aspergillus fumigatus isolate was identified to carry the mutation P88L in subunit HapE of the CCAAT-binding complex (CBC), a conserved eukaryotic transcription factor. Here, we define the mechanistic basis for resistance in this isolate by showing that the HapEP88L mutation interferes with the CBC’s ability to bend and sense CCAAT motifs. This failure leads to transcriptional derepression of the cyp51A gene, which encodes the target of azoles, the 14-α sterol demethylase Cyp51A, and ultimately causes drug resistance. In addition, we demonstrate that the CBC-associated transcriptional regulator HapX assists cyp51A repression in low-iron environments and that this iron-dependent effect is lost in the HapEP88L mutant. Altogether, these results indicate that the mutation HapEP88L confers increased resistance to azoles compared with wt A. fumigatus, particularly in low-iron clinical niches such as the lung.

DNA methylation reprogramming, TE derepression, and postzygotic isolation of nascent animal species

The genomic shock hypothesis stipulates that the stress associated with divergent genome admixture can cause transposable element (TE) derepression, which could act as a postzygotic isolation mechanism. TEs affect gene structure, expression patterns, and chromosome organization and may have deleterious consequences when released. For these reasons, they are silenced by heterochromatin formation, which includes DNA methylation. Here, we show that a significant proportion of TEs are differentially methylated between the “dwarf” (limnetic) and the “normal” (benthic) whitefish, two nascent species that diverged some 15,000 generations ago within the Coregonus clupeaformis species complex. Moreover, TEs are overrepresented among loci that were demethylated in hybrids, indicative of their transcriptional derepression. These results are consistent with earlier studies in this system that revealed TE transcriptional derepression causes abnormal embryonic development and death of hybrids. Hence, this supports a role of DNA methylation reprogramming and TE derepression in postzygotic isolation of nascent animal species.

BMI1–UBR5 axis regulates transcriptional repression at damaged chromatin

BMI1 is a component of the Polycomb Repressive Complex 1 (PRC1), which plays a key role in maintaining epigenetic silencing during development. BMI1 also participates in gene silencing during DNA damage response, but the precise downstream function of BMI1 in gene silencing is unclear. Here we identified the UBR5 E3 ligase as a downstream factor of BMI1. We found that UBR5 forms damage-inducible nuclear foci in a manner dependent on the PRC1 components BMI1, RNF1 (RING1a), and RNF2 (RING1b). Whereas transcription is repressed at UV-induced lesions on chromatin, depletion of the PRC1 members or UBR5 alone derepressed transcription elongation at these sites, suggesting that UBR5 functions in a linear pathway with PRC1 in inducing gene silencing at lesions. Mass spectrometry (MS) analysis revealed that UBR5 associates with BMI1 as well as FACT components SPT16 and SSRP1. We found that UBR5 localizes to the UV-induced lesions along with SPT16. We show that UBR5 ubiquitinates SPT16, and depletion of UBR5 or BMI1 leads to an enlargement of SPT16 foci size at UV lesions, suggesting that UBR5 and BMI1 repress SPT16 enrichment at the damaged sites. Consistently, depletion of the FACT components effectively reversed the transcriptional derepression incurred in the UBR5 and BMI1 KO cells. Finally, UBR5 and BMI1 KO cells are hypersensitive to UV, which supports the notion that faulty RNA synthesis at damaged sites is harmful to the cell fitness. Altogether, these results suggest that BMI1 and UBR5 repress the polymerase II (Pol II)-mediated transcription at damaged sites, by negatively regulating the FACT-dependent Pol II elongation.