August 12, 2022
One way to imagine DNA is as a busy road with a lot of competing traffic. Say, a small village in southern Italy…where someone must mediate conflicts between competing vehicles to avoid disaster.
It turns out that the “someone” in yeast cells is Sen1p. Two recent papers from associated groups describe the intriguing detail of how the Sen1p helicase plays this role for RNA polymerase III transcription. The paper by Aiello et al. in Molecular Cell shows how Sen1p regulates transcription-driven conflicts between the several machineries all engaged with DNA. In the related paper by Xie et al. in Science Advances, the authors show how the Sen1p helicase mediates “fail-safe” methods of transcription termination for RNA Pol III, thereby promoting efficiency and avoiding conflict with other pieces of machinery.
The key conflict preventing RNA Pol III from transcribing noncoding genes is with RNA Pol II, which is busy transcribing coding genes. Aiello et al. show how Sen1p has two strategies for mediating these conflicts, both of which involve interactions between Sen1p and the replisome. One involves temporary release of RNA Pol II from DNA while the other resolves genotoxic R-loops in nascent RNA. Both are critical for preventing genome instability.
In the related paper by Xie et al., the authors focus on how termination of transcription of noncoding genes by RNA Pol III is achieved, and the role that Sen1p plays in termination. They show how Sen1p can interact with all three polymerases and also with the other two subunits (Nrd1p and Nab3p) of the NRD1 snoRNA termination (NNS) complex. More specifically, they show by mutation and co-immunoprecipitation that it is the N-terminal domain (NTD) of Sen1p that interacts with RNA Pol III and the replisome.
The authors use metagene analysis of RNA Pol II distribution at mRNA-coding genes to show how Sen1p can promote the release of RNA Pol II to resolve transcription-replication conflicts (TRCs). They further show how the association of Sen1p with the replisome is required for limiting TRCs at the ribosomal replication fork barrier, and how this action appears redundant with that of RNases H. The cooperation and redundancy in this role are key means to protect genome stability.
Not only is Sen1p required for termination of RNA Pol III transcription, but the authors show how this function is independent of the NNS complex. Unlike resolution of conflicts between RNA Pol II and RNA Pol III, the termination function of Sen1p does not require the replisome.
They asked the question of whether Sen1p acts via the primary termination site for RNA Pol III or, rather, a backup secondary termination that catches errors (i.e., when RNA Pol III reads through a weak termination site). Termination for RNA Pol III employs a tract of T nucleotides (T-tract) in the nontemplate strand and these T-tracts can be relatively weak or strong. When T-tracts prove insufficient to stop the polymerase, Sen1p plays a role by means of secondary structures in nascent RNAs, which act as auxiliary cis-acting elements. This backup method is termed the “fail-safe transcription termination pathway.” The RNA secondary structures are not absolutely required for RNAPIII termination, but can function as auxiliary elements that bypass weak or defective termination signals.
Once more, it is the power of the yeast model that has allowed investigation to such exquisite molecular detail. That cells preserve genomic stability and avoid pile-ups amid so much traffic along DNA remains truly remarkable–even when we know more of how it works.
Categories: Research Spotlight
February 13, 2018
This is, of course, a mistake. The best person by far is Cinderella and she should not be ignored because of her position in life.
The RNA polymerases in a eukaryotic cell are similar except that one is the star and two are the scullery maids. RNA polymerase II (Pol II), the reader of protein encoding genes, gets most of the attention. Pol I and Pol III, the enzymes that make RNAs that aren’t usually translated into proteins, are mostly relegated to supporting roles.
Just as there were hints of Cinderella’s specialness, so too there were hints that perhaps Pol III plays an important role in longevity. That hint goes by the name of TORC1.
Previous work had shown that inhibiting TORC1 can increase the lifespan of beasts ranging from yeast to mice. Given that TORC1 is a strong activator of Pol III, Filer and coworkers set out to determine if inhibiting Pol III could do the same thing.
As any good researcher should, they started out by looking in our favorite model organism, the yeast Saccharomyces cerevisiae.
Pol III is pretty complicated—it is made up of 17 different subunits encoded by 17 different essential genes. These authors went after the largest subunit, C160, encoded by the RPO31 (RPC160) gene.
To control its level of expression, they fused an auxin-inducible degron to the protein. This fusion protein is degraded in the presence of indole-3-acetic acid (IAA) and E3 ubiquitin ligase from the rice Oryza sativa.
This means that there should be less Pol III in a cell when IAA is around. Western blot analysis confirmed that this was indeed the case.
The authors found that yeast with less Pol III had a longer chronological lifespan. In other words, each yeast cell survived longer. Having less Pol III did not significantly increase replicative lifespan though; they did not bud more often over their lifetime.
These authors did not get as significant a result when they targeted the largest subunit of Pol II, RPO21 (RPB220). The strain appeared to do better in the presence of IAA compared to the control strain, suggesting that targeting Pol II had a small effect on longevity; but it was nothing like was seen with Pol III.
When Filer and coworkers used RNA interference (RNAi) to knock down the amount of the largest subunit of Pol III, rpc-1, in C. elegans, the worm lived longer. And when they generated a strain of Drosophila which had only one working copy of dC53 (CG5147), a gene that encodes a Pol III subunit, these fruit flies lived longer too. Lowering the amount of Pol III is a good thing if you want to live longer!
Previous work had shown that the gut often controls longevity in C. elegans. These authors showed that the same holds true for Pol III reduction. Targeting just the Pol III in the gut was sufficient to give this little worm a longer life.
The same turned out to be true in the fruit fly. Using RNAi against two different Pol III subunits in the Drosophila gut was enough to have extend these flies’ lives. Targeting Pol III in the fat body did not have the same effect, and targeting Pol III in neuronal cells extended life only a little.
So reducing Pol III in the gut is good enough for flies and worms to live longer. In fact, reducing Pol III activity had as much effect on longevity as does inhibiting TORC1. Scientists ignore this hidden gem at their own risk!
Since Pol III is ignored, but dangerous for lifespan, perhaps a better analogy than Cinderella might be Sauron and the hobbits of the Shire from the Lord of the Rings Trilogy. Sauron ignores the hobbits who hold the key (or the ring) to Sauron’s shortened lifespan.
If Sauron had paid attention, he’d still be lording it over Middle Earth at the end of the third book. Just as we might live longer if we start paying more attention to that hidden danger—excessive Pol III activity, possibly in the gut.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
March 05, 2013
Cancer often gets going with chromosome instability. Basically a cell gets a mutation that causes its chromosomes to mutate at a higher rate. Now it and any cells that come from it build mutations faster and faster until they hit on the right combination to make the cell cancerous. An accelerating avalanche of mutations has led to cancer.
There are plenty of obvious candidates for the genes that start these avalanches: genes like those involved in segregating chromosomes and repairing DNA, for example. But there are undoubtedly sleeper genes that no one has really thought of. In a new study out in GENETICS, Minaker and coworkers have used the yeast S. cerevisiae to identify three of these genes — GPN1 (previously named NPA3), GPN2, and GPN3.
A mutation in any one of these genes leads to chromosomal problems. For example, mutations in GPN1 and GPN2 cause defects in sister chromatid cohesion and mutations in GPN3 confer a visible chromosome transmission defect. All of the mutants also show increased sensitivity to hydroxyurea and ultraviolet light, two potent mutagens. And if two of the genes are mutated at once, these defects become more severe. Clearly, mutating GPN1, GPN2, and/or GPN3 leads to an increased risk for even more mutations!
What makes this surprising is what these genes actually do in a cell. They are responsible for getting RNA polymerase II (RNAPII) and RNA polymerase III (RNAPIII) into the nucleus and assembled properly. This was known before for GPN1, but here the authors show that in gpn2 and gpn3 mutants, RNAPII and RNAPIII subunits also fail to get into the nucleus. Genetic and physical interactions between all three GPN proteins suggest that they work together in overlapping ways to get enough RNAPII and RNAPIII chugging away in the nucleus.
So it looks like having too little RNAPII and RNAPIII in the nucleus causes chromosome instability. This is consistent with previous work that shows that mutations in many of the RNAPII subunits have similar effects. Still, these genes would not be the first ones most scientists would look at when trying to find causes of chromosomal instability. Score another point for unbiased screens in yeast leading to a better understanding of human disease.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
April 23, 2012
SGD has added a new mix of data tracks to our GBrowse genome viewer from seven publications covering transcriptome exploration via tiling microarrays (David et al. 2006), genomic occupancy of RNA polymerase II and III and associated factors (Kim et al. 2010; Ghavi-Helm 2008), 3′ end processing (Johnson et al. 2011), histone H2BK123 monoubiquitination (Schulze et al. 2011) and high-resolution ChIP by a novel method called ChIP-exo (Rhee et al. 2011; Rhee et al. 2012). Download data tracks, metadata and supplementary data by clicking on the ‘?’ icon on each data track within GBrowse or directly from the SGD downloads page. We welcome new data submissions pre- or post-publication and invite authors to work with us to integrate their data into our GBrowse and PBrowse viewers. Please contact us if you are interested in participating or have questions and comments. Happy browsing!
Categories: New Data