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Telomerase elongates telomeres to maintain chromosome stability in most eukaryotes. Despite extensive studies across eukaryotic kingdoms, the telomerase holoenzyme in arthropods remains poorly understood. In this study, we purify the telomerase ribonucleoprotein complex from the lepidopteran insectSpodoptera frugiperda(fall armyworm) and identify a copurified 135-nucleotide telomerase RNA (TR) component. This miniatureS. frugiperdaTR (sfTR), the smallest TR known to date, retains a universal pseudoknot structure and a structurally defined template. Despite its small size, sfTR assembles with the recombinantS. frugiperdatelomerase reverse transcriptase (sfTERT) protein in vivo to reconstitute telomerase activity for the synthesis of insect telomeric DNA repeats (TTAGG)n. The sfTR gene, like other animal TR genes, features an snRNA-type RNA polymerase II promoter. Uniquely, the sfTR transcript harbors a 5′-7-methylguanosine (M⁷G) cap, as opposed to the more typical snRNA-type 2,2,7-trimethylguanosine (TMG) cap. The difference in 5′-cap is likely because sfTR lacks the H/ACA snoRNA biogenesis domain necessary for cap hypermethylation. Moreover, sfTR also lacks the CR4/5 regulatory domain that is indispensable in vertebrate TRs for telomerase activity. This degenerate sfTR complements an enigmatic sfTERT that is missing certain telomerase-specific elements yet catalytically active in the absence of sfTR. Thus, insects have evolved a simplified telomerase, consisting of a small noncoding RNA that retains only minimal attributes essential for telomerase function. The simplified insect telomerase demonstrates a plausible evolutionary pathway for the emergence of telomerase ribonucleoprotein complex, arising from an ancient reverse transcriptase associated with a simple templating RNA component in early eukaryotes. 

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.