Regulation of tRNA synthesis by posttranslational modifications of RNA polymerase III subunits

Intragenic tRNA-promoted R-loops orchestrate transcription interference for plant oxidative stress responses

Eukaryotic genomes are transcribed by at least three RNA polymerases, RNAPI, II, and III. Co-transcriptional R-loops play diverse roles in genome regulation and maintenance. However, little is known about how R-loops regulate transcription interference, the transcriptional event that is caused by different RNA polymerases transcribing the same genomic templates. Here, we established that the intragenic transfer RNA (tRNA) genes can promote sense R-loop enrichment (named intra-tR-loops) in Arabidopsis thaliana, and found that intra-tR-loops are decreased in an RNAPIII mutant, NUCLEAR RNA POLYMERASE C, SUBUNIT 7(nrpc7-1). NRPC7 is co-localized with RNAPIIS2P at intragenic tRNA genes and interferes with RNAPIIS2P elongation. Conversely, the binding of NRPC7 at intragenic tRNA genes is increased following inhibition of RNAPII elongation. The transcription of specific tRNA host genes is inhibited by RNAPIII, and the inhibition of tRNA host genes is intra-tR-loop dependent. Moreover, alleviating the inhibition of tRNAPro-induced intra-tR-loops on its host gene AtNUDX1 promotes oxidative stress tolerance in A. thaliana. Our work suggests intra-tR-loops regulate host gene expression by modulating RNA polymerases interference.

Desumoylation of RNA polymerase III lies at the core of the Sumo stress response in yeast

Post-translational modification by small ubiquitin-like modifier (Sumo) regulates many cellular processes, including the adaptive response to various types of stress, referred to as the Sumo stress response (SSR). However, it remains unclear whether the SSR involves a common set of core proteins regardless of the type of stress or whether each particular type of stress induces a stress-specific SSR that targets a unique, largely nonoverlapping set of Sumo substrates. In this study, we used MS and a Gene Ontology approach to identify differentially sumoylated proteins during heat stress, hyperosmotic stress, oxidative stress, nitrogen starvation, and DNA alkylation in Saccharomyces cerevisiae cells. Our results indicate that each stress triggers a specific SSR signature centered on proteins involved in transcription, translation, and chromatin regulation. Strikingly, whereas the various stress-specific SSRs were largely nonoverlapping, all types of stress tested here resulted in desumoylation of subunits of RNA polymerase III, which correlated with a decrease in tRNA synthesis. We conclude that desumoylation and subsequent inhibition of RNA polymerase III constitutes the core of all stress-specific SSRs in yeast.

Cdk1 gates cell cycle-dependent tRNA synthesis by regulating RNA polymerase III activity

tRNA genes are transcribed by RNA polymerase III (RNAPIII). During recent years it has become clear that RNAPIII activity is strictly regulated by the cell in response to environmental cues and the homeostatic status of the cell. However, the molecular mechanisms that control RNAPIII activity to regulate the amplitude of tDNA transcription in normally cycling cells are not well understood. Here, we show that tRNA levels fluctuate during the cell cycle and reveal an underlying molecular mechanism. The cyclin Clb5 recruits the cyclin dependent kinase Cdk1 to tRNA genes to boost tDNA transcription during late S phase. At tDNA genes, Cdk1 promotes the recruitment of TFIIIC, stimulates the interaction between TFIIIB and TFIIIC, and increases the dynamics of RNA polymerase III in vivo. Furthermore, we identified Bdp1 as a putative Cdk1 substrate in this process. Preventing Bdp1 phosphorylation prevented cell cycle-dependent recruitment of TFIIIC and abolished the cell cycle-dependent increase in tDNA transcription. Our findings demonstrate that under optimal growth conditions Cdk1 gates tRNA synthesis in S phase by regulating the RNAPIII machinery, revealing a direct link between the cell cycle and RNAPIII activity.

The multi-faceted regulation of nuclear tRNA gene transcription

Transfer RNAs are among the most ancient molecules of life on earth. Beyond their crucial role in protein synthesis as carriers of amino acids, they are also important players in a plethora of other biological processes. Many debates in term of biogenesis, regulation and function persist around these fascinating non-coding RNAs. Our review focuses on the first step of their biogenesis in eukaryotes, i.e. their transcription from nuclear genes. Numerous and complementary ways have emerged during evolution to regulate transfer RNA gene transcription. Here, we will summarize the different actors implicated in this process: cis-elements, trans-factors, genomic contexts, epigenetic environments and finally three-dimensional organization of nuclear genomes. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1099-1108, 2019.

Robust repression of tRNA gene transcription during stress requires protein arginine methylation

Protein arginine methylation is an important means by which protein function can be regulated. In the budding yeast, this modification is catalyzed by the major protein arginine methyltransferase Hmt1. Here, we provide evidence that the Hmt1-mediated methylation of Rpc31, a subunit of RNA polymerase III, plays context-dependent roles in tRNA gene transcription: under conditions optimal for growth, it positively regulates tRNA gene transcription, and in the setting of stress, it promotes robust transcriptional repression. In the context of stress, methylation of Rpc31 allows for its optimal interaction with RNA polymerase III global repressor Maf1. Interestingly, mammalian Hmt1 homologue is able to methylate one of Rpc31's human homologue, RPC32β, but not its paralogue, RPC32α. Our data led us to propose an efficient model whereby protein arginine methylation facilitates metabolic economy and coordinates protein-synthetic capacity.

The yeast Ty1 retrotransposon requires components of the nuclear pore complex for transcription and genomic integration

Nuclear pore complexes (NPCs) orchestrate cargo between the cytoplasm and nucleus and regulate chromatin organization. NPC proteins, or nucleoporins (Nups), are required for human immunodeficiency virus type 1 (HIV-1) gene expression and genomic integration of viral DNA. We utilize the Ty1 retrotransposon of Saccharomyces cerevisiae (S. cerevisiae) to study retroviral integration because retrotransposons are the progenitors of retroviruses and have conserved integrase (IN) enzymes. Ty1-IN targets Ty1 elements into the genome upstream of RNA polymerase (Pol) III transcribed genes such as transfer RNA (tRNA) genes. Evidence that S. cerevisiae tRNA genes are recruited to NPCs prompted our investigation of a functional role for the NPC in Ty1 targeting into the genome. We find that Ty1 mobility is reduced in multiple Nup mutants that cannot be accounted for by defects in Ty1 gene expression, cDNA production or Ty1-IN nuclear entry. Instead, we find that Ty1 insertion upstream of tRNA genes is impaired. We also identify Nup mutants with wild type Ty1 mobility but impaired Ty1 targeting. The NPC nuclear basket, which interacts with chromatin, is required for both Ty1 expression and nucleosome targeting. Deletion of components of the NPC nuclear basket causes mis-targeting of Ty1 elements to the ends of chromosomes.