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1. Ustilago maydis telomere protein Pot1 harbors an extra N-terminal OB fold and regulates homology-directed DNA repair factors in a dichotomous and context-dependent manner

   https://doi.org/10.1371/journal.pgen.1010182  

   Zahid, Syed ; Aloe, Sarah ; Sutherland, Jeanette H. ; Holloman, William K. ; Lue, Neal F. ( May 2022 , PLOS Genetics)

   Zhou, Jin-Qiu (Ed.)

   The telomere G-strand binding protein Pot1 plays multifaceted roles in telomere maintenance and protection. We examined the structure and activities of Pot1 in Ustilago maydis , a fungal model that recapitulates key features of mammalian telomere regulation. Compared to the well-characterized primate and fission yeast Pot1 orthologs, Um Pot1 harbors an extra N-terminal OB-fold domain (OB-N), which was recently shown to be present in most metazoans. Um Pot1 binds directly to Rad51 and regulates the latter’s strand exchange activity. Deleting the OB-N domain, which is implicated in Rad51-binding, caused telomere shortening, suggesting that Pot1-Rad51 interaction facilitates telomere maintenance. Depleting Pot1 through transcriptional repression triggered growth arrest as well as rampant recombination, leading to multiple telomere aberrations. In addition, telomere repeat RNAs transcribed from both the G- and C-strand were dramatically up-regulated, and this was accompanied by elevated levels of telomere RNA-DNA hybrids. Telomere abnormalities of pot1 -deficient cells were suppressed, and cell viability was restored by the deletion of genes encoding Rad51 or Brh2 (the BRCA2 ortholog), indicating that homology-directed repair (HDR) proteins are key mediators of telomere aberrations and cellular toxicity. Together, these observations underscore the complex physical and functional interactions between Pot1 and DNA repair factors, leading to context-dependent and dichotomous effects of HDR proteins on telomere maintenance and protection. 

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2. Duplex Telomere-Binding Proteins in Fungi With Canonical Telomere Repeats: New Lessons in the Rapid Evolution of Telomere Proteins

   https://doi.org/10.3389/fgene.2021.638790  

   Lue, Neal F. ( February 2021 , Frontiers in Genetics)

   null (Ed.)

   The telomere protein assemblies in different fungal lineages manifest quite profound structural and functional divergence, implying a high degree of flexibility and adaptability. Previous comparative analyses of fungal telomeres have focused on the role of telomere sequence alterations in promoting the evolution of corresponding proteins, particularly in budding and fission yeast. However, emerging evidence suggests that even in fungi with the canonical 6-bp telomere repeat unit, there are significant remodeling of the telomere assembly. Indeed, a new protein family can be recruited to serve dedicated telomere functions, and then experience subsequent loss in sub-branches of the clade. An especially interesting example is the Tay1 family of proteins, which emerged in fungi prior to the divergence of basidiomycetes from ascomycetes. This relatively recent protein family appears to have acquired its telomere DNA-binding activity through the modification of another Myb-containing protein. Members of the Tay1 family evidently underwent rather dramatic functional diversification, serving, e.g., as transcription factors in fission yeast while acting to promote telomere maintenance in basidiomycetes and some hemi-ascomycetes. Remarkably, despite its distinct structural organization and evolutionary origin, a basidiomycete Tay1 appears to promote telomere replication using the same mechanism as mammalian TRF1, i.e., by recruiting and regulating Blm helicase activity. This apparent example of convergent evolution at the molecular level highlight the ability of telomere proteins to acquire new interaction targets. The remarkable evolutionary history of Tay1 illustrates the power of protein modularity and the facile acquisition of nucleic acid/protein-binding activity to promote telomere flexibility. 

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3. Biogenesis of telomerase RNA from a protein-coding mRNA precursor

   https://doi.org/10.1073/pnas.2204636119  

   Logeswaran, Dhenugen ; Li, Yang ; Akhter, Khadiza ; Podlevsky, Joshua D. ; Olson, Tamara L. ; Forsberg, Katherine ; Chen, Julian J.-L. ( October 2022 , Proceedings of the National Academy of Sciences)

   Telomerase is a eukaryotic ribonucleoprotein (RNP) enzyme that adds DNA repeats onto chromosome ends to maintain genomic stability and confer cellular immortality in cancer and stem cells. The telomerase RNA (TER) component is essential for telomerase catalytic activity and provides the template for telomeric DNA synthesis. The biogenesis of TERs is extremely divergent across eukaryotic kingdoms, employing distinct types of transcription machinery and processing pathways. In ciliates and plants, TERs are transcribed by RNA polymerase III (Pol III), while animal and ascomycete fungal TERs are transcribed by RNA Pol II and share biogenesis pathways with small nucleolar RNA (snoRNA) and small nuclear RNA (snRNA), respectively. Here, we report an unprecedented messenger RNA (mRNA)-derived biogenesis pathway for the 1,291 nucleotide TER from the basidiomycete fungus Ustilago maydis . The U. maydis TER ( Um TER) contains a 5′-monophosphate, distinct from the 5′ 2,2,7-trimethylguanosine (TMG) cap common to animal and ascomycete fungal TERs. The mature Um TER is processed from the 3′-untranslated region (3′-UTR) of a larger RNA precursor that possesses characteristics of mRNA including a 5′ 7-methyl-guanosine (m 7 G) cap, alternative splicing of introns, and a poly(A) tail. Moreover, this mRNA transcript encodes a protein called Early meiotic induction protein 1 (Emi1) that is conserved across dikaryotic fungi. A recombinant Um TER precursor expressed from an mRNA promoter is processed correctly to yield mature Um TER, confirming an mRNA-processing pathway for producing TER. Our findings expand the plethora of TER biogenesis mechanisms and demonstrate a pathway for producing a functional long noncoding RNA from a protein-coding mRNA precursor. 

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4. Telomere Roles in Fungal Genome Evolution and Adaptation

   https://doi.org/10.3389/fgene.2021.676751  

   Rahnama, Mostafa ; Wang, Baohua ; Dostart, Jane ; Novikova, Olga ; Yackzan, Daniel ; Yackzan, Andrew ; Bruss, Haley ; Baker, Maray ; Jacob, Haven ; Zhang, Xiaofei ; et al ( August 2021 , Frontiers in Genetics)

   Telomeres form the ends of linear chromosomes and usually comprise protein complexes that bind to simple repeated sequence motifs that are added to the 3′ ends of DNA by the telomerase reverse transcriptase (TERT). One of the primary functions attributed to telomeres is to solve the “end-replication problem” which, if left unaddressed, would cause gradual, inexorable attrition of sequences from the chromosome ends and, eventually, loss of viability. Telomere-binding proteins also protect the chromosome from 5′ to 3′ exonuclease action, and disguise the chromosome ends from the double-strand break repair machinery whose illegitimate action potentially generates catastrophic chromosome aberrations. Telomeres are of special interest in the blast fungus, Pyricularia , because the adjacent regions are enriched in genes controlling interactions with host plants, and the chromosome ends show enhanced polymorphism and genetic instability. Previously, we showed that telomere instability in some P. oryzae strains is caused by novel retrotransposons (MoTeRs) that insert in telomere repeats, generating interstitial telomere sequences that drive frequent, break-induced rearrangements. Here, we sought to gain further insight on telomeric involvement in shaping Pyricularia genome architecture by characterizing sequence polymorphisms at chromosome ends, and surrounding internalized MoTeR loci (relics) and interstitial telomere repeats. This provided evidence that telomere dynamics have played historical, and likely ongoing, roles in shaping the Pyricularia genome. We further demonstrate that even telomeres lacking MoTeR insertions are poorly preserved, such that the telomere-adjacent sequences exhibit frequent presence/absence polymorphism, as well as exchanges with the genome interior. Using TERT knockout experiments, we characterized chromosomal responses to failed telomere maintenance which suggested that much of the MoTeR relic-/interstitial telomere-associated polymorphism could be driven by compromised telomere function. Finally, we describe three possible examples of a phenomenon known as “Adaptive Telomere Failure,” where spontaneous losses of telomere maintenance drive rapid accumulation of sequence polymorphism with possible adaptive advantages. Together, our data suggest that telomere maintenance is frequently compromised in Pyricularia but the chromosome alterations resulting from telomere failure are not as catastrophic as prior research would predict, and may, in fact, be potent drivers of adaptive polymorphism. 

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5. Smut infection of perennial hosts: the genome and the transcriptome of the Brassicaceae smut fungus Thecaphora thlaspeos reveal functionally conserved and novel effectors

   https://doi.org/10.1111/nph.15692  

   Courville, Kaitlyn J. ; Frantzeskakis, Lamprinos ; Gul, Summia ; Haeger, Natalie ; Kellner, Ronny ; Heßler, Natascha ; Day, Brad ; Usadel, Björn ; Gupta, Yogesh K. ; van Esse, H. Peter ; et al ( February 2019 , New Phytologist)

   Biotrophic fungal plant pathogens can balance their virulence and form intricate relationships with their hosts. Sometimes, this leads to systemic host colonization over long time scales without macroscopic symptoms. However, how plant‐pathogenic endophytes manage to establish their sustained systemic infection remains largely unknown.

   Here, we present a genomic and transcriptomic analysis ofThecaphora thlaspeos. This relative of the well studied grass smutUstilago maydisis the only smut fungus adapted to Brassicaceae hosts. Its ability to overwinter with perennial hosts and its systemic plant infection including roots are unique characteristics among smut fungi.

   TheT. thlaspeosgenome was assembled to the chromosome level. It is a typical smut genome in terms of size and genome characteristics.In silicoprediction of candidate effector genes revealed common smut effector proteins and unique members. For three candidates, we have functionally demonstrated effector activity. One of these,TtTue1, suggests a potential link to cold acclimation. On the plant side, we found evidence for a typical immune response as it is present in other infection systems, despite the absence of any macroscopic symptoms during infection.

   Our findings suggest thatT. thlaspeosdistinctly balances its virulence during biotrophic growth ultimately allowing for long‐lived infection of its perennial hosts.

    

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