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. 2011 Jan 7;41(1):21-32.
doi: 10.1016/j.molcel.2010.12.007.

Yeast Sen1 helicase protects the genome from transcription-associated instability

Hannah E Mischo  1 , Belén Gómez-González, Pawel Grzechnik, Ana G Rondón, Wu Wei, Lars Steinmetz, Andrés Aguilera, Nick J Proudfoot
Affiliations

Yeast Sen1 helicase protects the genome from transcription-associated instability

Hannah E Mischo et al. Mol Cell. .

Abstract

Sen1 of S. cerevisiae is a known component of the NRD complex implicated in transcription termination of nonpolyadenylated as well as some polyadenylated RNA polymerase II transcripts. We now show that Sen1 helicase possesses a wider function by restricting the occurrence of RNA:DNA hybrids that may naturally form during transcription, when nascent RNA hybridizes to DNA prior to its packaging into RNA protein complexes. These hybrids displace the nontranscribed strand and create R loop structures. Loss of Sen1 results in transient R loop accumulation and so elicits transcription-associated recombination. SEN1 genetically interacts with DNA repair genes, suggesting that R loop resolution requires proteins involved in homologous recombination. Based on these findings, we propose that R loop formation is a frequent event during transcription and a key function of Sen1 is to prevent their accumulation and associated genome instability.

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Figures

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Graphical abstract
Figure 1
Figure 1
Sen1 Helicase Is Required for Transcription Termination but Not Transcript 3′ Processing (A) Top: pKGG and positions of ssM13 probes (1–7). Domain structure of Sen1. Middle: representative TRO filters of WT and sen1-1 cells cotransformed with KGG and empty vector, Sen1(323), or Sen1(1212) constructs. Transformants were grown for 150 min at 37°C before TRO. Bottom: quantification based on four repeat experiments. (B) RNA isolated from the inocules used for TRO analysis probed for KGG and endogenous PMA1 mRNA, as well as Pol III transcript scR1. Bottom: quantification of four repeat experiments. (C) In vitro cleavage and polyadenylation assays performed with extracts from WT and mutant cells grown for 150 or 90 min (rna14-1) at 37°C with CYC1 3′ pA as substrate. Positions of uncleaved, polyadenylated, cleaved, and 3′ end cleavage product are indicated. All error bars represent the standard deviation (SD). See also Figure S1.
Figure 2
Figure 2
sen1-1 Are Hyperrecombinogenic (A) Recombination substrates LNA and LNAT. Transformants were grown for 3–4 days at 30°C. Recombination generates a functional LEU2, allowing selection of recombinants on leu-deficient plates. Quantification of recombinants formed from six colonies of four to six transformants is presented. (B) Recombinants formed in WT and sen1-1 transformed with L and LYΔNS containing homologous repeats separated by 39 or 3900 nt. (C) As in (B), with the LLacZ and LPHO5 substrates under control of either LEU2 or glucose-repressed GAL1 promoters to stimulate high or low expression. (D) Diagram and recombination quantification of chromosomal construct crossed into the WT and sen1-1. All error bars represent the SD. See also Figure S2.
Figure 3
Figure 3
sen1-1 Cells Form a Substrate for AID and RNase H (A) AID overexpression increases sen1-1 TAR. pGLG recombinants forming GFP were counted after 12–16 hr growth at 30°C by FACS. AID coexpression increases GFP-positive cells in both WT and sen1-1 strains (p = 0.018, Wilcoxon test). (B) Coexpression of AID and pLAUR-induced mutations within URA3 in pLAUR were scored as 5-FOA resistant. (C) URA3 sequence from mutants was amplified and sequenced. The frequency of point mutations on either strand is depicted graphically (see Figure S3 for more detail). (D) Effect of galactose-induced expression from plasmids pRNH201 (coding RNase H RNH201), pYsen1 (aa 1281–2231 of Sen1 cloned into pYES2), or pYES2 alone on the recombination frequencies in WT and sen1-1 cells produced by the LLacZ system. Note that double selection and growth on galactose reduces the sen1-1 viability and therefore recombination frequencies as compared to data in Figure 2C. (E) Recombination frequency analysis in the THO-complex mutant mft1Δ with the GLG recombination substrate and AID to increase recombination rates. Overexpression of Sen1 helicase reduces recombination frequency. All error bars represent the SD. See also Figure S3.
Figure 4
Figure 4
R Loops Form in sen1-1 Cells (A) DIP analysis on pGLLacZ in sen1-1, hpr1Δ, and WT cells with antibody against RNA:DNA hybrids (S9.6). Coimmunoprecipitated DNA was detected by real-time qPCR. Inocules were grown in raffinose, induced with galactose for 1 hr at 37°C, and successively repressed at 37°C by addition of 4% glucose. (B) Reverse transcription (RT) of RNA isolated from genomic DNA preparations after treatment with RNase H. The levels of RNase H resistant RNA were measured after RT with P5 and P7 primers followed by PCR with P5 amplicon (as shown on the gene map). Signals obtained were free from DNA contamination based on minus RT controls. Left: representative gel. Right: quantification of RT normalized triplicate repeats by real-time qRT-PCR, further normalized to P5 amplicon signal obtained from non-RNase H-digested samples. All error bars represent the SD. See also Figure S4.
Figure 5
Figure 5
sen1-1 Shows Synthetic Genetic Interaction with DNA Damage Repair Genes (A) Synthetic interactions between sen1-1 and MRX gene mutations: mre11Δ, rad50Δ. Also shown are synthetic interactions between sen1-1 and HR gene mutants sgs1Δ and rad52Δ. White boxes indicate spores that carry both mutations. (B) Analysis of HU sensitivity of double mutants grown at 25°C. Growth was compared on YPAD plates ± 50 mM HU (10 mM ∗ or 100 mM ∗∗ as indicated). See also Figure S5 and Table S4.
Figure 6
Figure 6
DNA Repair Foci in sen1-1 Nuclei (A) Time course of WT and sen1-1 grown at log phase and shifted to 30° or 37°C. At indicated time points aliquots were spotted on microscope slides and foci-containing cells scored based on 300 cells. Representative pictures (top left) and quantification of 3-5 repeats are shown (bottom left). Simultaneously isolated RNA was analyzed by Northern Blot against SNR13 to show accumulation of bi-cistronic SNR13-TRS31 transcript in sen1-1. (B) WT and sen1-1 cells transformed with pWJ144 and pYSen1 grown in raffinose and shifted to 30°C for 1hr, before Sen1 helicase fragment expression was induced or repressed by addition of 2% galactose or 2% glucose to the medium respectively. Only WT in galactose shown and foci containing cells scored as in A. (C) sen1-1 rpb1-1 cells transformed with pWJ144 were grown in logarithmic phase at 25°C and then shifted to 37°C. Shutoff of transcription results in decrease of Rad52 foci, either by reduced accumulation or Rad52 turnover. All error bars represent the SD.
Figure 7
Figure 7
Cotranscriptional Functions of Sen1 (A) Model for Sen1 cotranscriptional function especially in termination regions. (B) Model for R loop accumulation in sen1-1 showing three ways they may elicit HR: processing of nicks in ssDNA, ssDNA recognition, and collapse of colliding replication forks.
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References

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