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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. 

Telomere maintenance is a fundamental cellular process conserved across all eukaryotic lineages. Although plants and animals diverged over 1.5 billion years ago, lessons learned from plants continue to push the boundaries of science, revealing detailed molecular mechanisms in telomere biology with broad implications for human health, aging biology, and stress responses. Recent studies of plant telomeres have unveiled unexpected divergence in telomere sequence and architecture, and the proteins that engage telomeric DNA and telomerase. The discovery of telomerase RNA components in the plant kingdom and some algae groups revealed new insight into the divergent evolution and the universal core of telomerase across major eukaryotic kingdoms. In addition, resources cataloging the abundant natural variation in Arabidopsis thaliana, maize (Zea mays), and other plants are providing unparalleled opportunities to understand the genetic networks that govern telomere length polymorphism and, as a result, are uncovering unanticipated crosstalk between telomeres, environmental factors, organismal fitness, and plant physiology. Here we recap current advances in plant telomere biology and put this field in perspective relative to telomere and telomerase research in other eukaryotic lineages.

 

Telomerase is essential for maintaining telomere integrity. Although telomerase function is widely conserved, the integral telomerase RNA (TR) that provides a template for telomeric DNA synthesis has diverged dramatically. Nevertheless, TR molecules retain 2 highly conserved structural domains critical for catalysis: a template-proximal pseudoknot (PK) structure and a downstream stem-loop structure. Here we introduce the authentic TR from the plant Arabidopsis thaliana , called AtTR, identified through next-generation sequencing of RNAs copurifying with Arabidopsis TERT. This RNA is distinct from the RNA previously described as the templating telomerase RNA, AtTER1. AtTR is a 268-nt Pol III transcript necessary for telomere maintenance in vivo and sufficient with TERT to reconstitute telomerase activity in vitro. Bioinformatics analysis identified 85 AtTR orthologs from 3 major clades of plants: angiosperms, gymnosperms, and lycophytes. Through phylogenetic comparisons, a secondary structure model conserved among plant TRs was inferred and verified using in vitro and in vivo chemical probing. The conserved plant TR structure contains a template-PK core domain enclosed by a P1 stem and a 3′ long-stem P4/5/6, both of which resemble a corresponding structural element in ciliate and vertebrate TRs. However, the plant TR contains additional stems and linkers within the template-PK core, allowing for expansion of PK structure from the simple PK in the smaller ciliate TR during evolution. Thus, the plant TR provides an evolutionary bridge that unites the disparate structures of previously characterized TRs from ciliates and vertebrates.