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RNA-Mediated Trans-Communication Can Establish Paramutation at the B1 Locus in Maize

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RNA-Mediated Trans-Communication Can Establish Paramutation at the B1 Locus in Maize RNA-mediated trans-communication can establish paramutation at the b1 locus in maize Mario Arteaga-Vazqueza, Lyudmila Sidorenkoa, Fernando A. Rabanala,b, Roli Shrivistavac, Kan Nobutac, Pamela J. Greenc, Blake C. Meyersc, and Vicki L. Chandlera,1 aBIO5 Institute and Department of Plant Sciences, University of Arizona, Tucson, AZ 85721; bDepartamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, 62210 Cuernavaca, Morelos, Mexico; and cDepartment of Plant and Soil Sciences and Delaware Biotechnology Institute, University of Delaware, Newark, DE 19711 Contributed by Vicki L. Chandler, June 6, 2010 (sent for review April 23, 2010) Paramutation is the epigenetic transfer of information between a subset of transposable elements (TEs), transgenes, and several alleles that leads to the heritable change of expression of one allele. non-TE genes (7, 13–18). Paramutation at the b1 locus in maize requires seven noncoding tan- The b1 gene is one of only two genes for which the sequences dem repeat (b1TR) sequences located ∼100 kb upstream of the tran- mediating paramutation have been defined (12, 19, 20). The key scription start site of b1, and mutations in several genes required for sequences required for b1 paramutation are seven b1TR units b1TR paramutation implicate an RNA-mediated mechanism. The mediator (each of the units is 853 bp in length) of noncoding DNA ∼ b1 of paramutation (mop1) gene, which encodes a protein closely re- located 100 kb upstream of the transcription start site (20, 21). This sequence is unique to this location within the maize lated to RNA-dependent RNA polymerases, is absolutely required for B-I B′ paramutation. Herein, we investigate the potential function of mop1 genome, and both and carry seven tandem repeats, whereas b1TR alleles that do not undergo paramutation have a single copy of the and the siRNAs that are produced from the sequences. Produc- B-I B′ tion of siRNAs from the b1TR sequences depends on a functional repeat unit (20, 21). and are epialleles; that is, they have identical DNA sequences but show distinct patterns of DNA mop1 gene, but transcription of the repeats is not dependent on methylation and chromatin structure within the tandem repeats mop1. Further nuclear transcription assays suggest that the b1TR (20, 22–24). Generation of an allelic series with different numbers sequences are likely transcribed predominantly by RNA polymerase b1TR of repeats demonstrated that multiple repeats are required for II. To address whether production of -siRNAs correlated with paramutation (20). paramutation, we examined siRNA production in alleles that cannot The tandem repeats mediate enhancer activity that functions undergo paramutation. Alleles that cannot participate in paramuta- in cis to increase transcription from the b1 gene when in the B-I b1TR b1TR tion also produce -siRNAs, suggesting that -siRNAs are state (20), potentially through a long-distance looping mechanism not sufficient for paramutation in the tissues analyzed. However, because the tandem repeats interact with the transcription start site when b1TR-siRNAs are produced from a transgene expressing a hair- of the b1 gene differentially in B-I vs. B′ (23). The molecular nature pin RNA, b1 paramutation can be recapitulated. We hypothesize of the genes required for paramutation strongly suggests that an that either the b1TR-siRNAs or the dsRNA template mediates the RNA-dependent mechanism is critical for paramutation. Consis- trans-communication between the alleles that establishes paramuta- tent with this idea, transcription assays have demonstrated that the tion. In addition, we uncovered a role for mop1 in the biogenesis of repeats are transcribed in B-I and B′ as well as in alleles that do not a subset of microRNAs (miRNAs) and show that it functions at the undergo paramutation (4). Bidirectional transcription potentially level of production of the primary miRNA transcripts. generates dsRNA, the trigger molecule in a number of transcrip- tional and posttranscriptional gene regulation mechanisms that interchromosomal | transfer | epigenetic | information | trans-generational involve the processing of dsRNA into different classes of regula- tory small RNAs (25, 26). Recent experiments have shown that b1TR B′ aramutation is an interaction between alleles that leads to a siRNAs are produced from the sequences in (27). Pheritable change of expression of one allele. One of the most In this study, we used transcription assays, deep sequencing of intensively studied examples of paramutation is at the b1 locus in small RNA libraries, and Northern blot analysis to investigate the potential steps in siRNA biogenesis where mop1 may function and maize (1), which encodes a transcription factor that activates the b1TR purple anthocyanin biosynthetic pathway (2). There are two alleles whether production of -siRNAs correlates with paramuta- involved in b1 paramutation, the highly transcribed and darkly tion. We also test whether DNA-dependent RNA polymerases are b1TR pigmented B-I allele and the lightly pigmented B′ allele that has mediating transcription from the sequences and investigate mop1 much lower transcription. When B-I and B′ are crossed together, alterations in microRNAs (miRNAs) in a mutant. paramutation always occurs: B-I is always changed into B′ (3). Several genes required for paramutation have been identified Results through forward genetic screens. The mediator of paramutation The mop1-1 Mutation Does Not Reduce Transcription of the b1TR (mop) genes (1, 4–6) and the required to maintain repression (rmr) Sequences. Previously, we showed that the b1TR sequences are genes (6, 8–10) have been isolated using the b1 and pl1 systems, transcribed on both strands using nuclear run-on experiments, respectively. To date, all characterized genes required for para- which should monitor either DNA-templated or RNA-templated mutation identified through forward genetic screens encode transcription in nuclei. To test whether the transcription we ob- proteins that have been associated with siRNA biogenesis in other species (1). Recently, a protein that binds to the b1 tandem repeat b1TR) fi ( sequences was identi ed, and expression of this protein Author contributions: M.A.-V., L.S., B.C.M., and V.L.C. designed research; M.A.-V., L.S., as a transgene can establish a paramutagenic state in B-I (11). The F.A.R., R.S., K.N., P.J.G., and B.C.M. performed research; M.A.-V., L.S., F.A.R., R.S., K.N., mop1 gene, which is the focus of this study, encodes a protein with B.C.M., and V.L.C. analyzed data; and M.A.-V., L.S., and V.L.C. wrote the paper. high similarity to RNA-dependent RNA polymerases (RDRs) The authors declare no conflict of interest. and is the predicted ortholog of RDR2 in Arabidopsis thaliana Data deposition: The sequences reported in this paper have been deposited in the Gene (Arabidopsis) (4, 5, 7). Activity of mop1 is required for para- Expression Omnibus (GEO) database, www.ncbi.nlm.nih.gov/geo (accession nos. mutation at the b1 locus and other loci (5, 6, 12), and it is required GSM306487 and GSM306488). to maintain the silent B′ state (5). Similar to Arabidopsis RDR2, 1To whom correspondence should be addressed. E-mail: [email protected]. mop1 is required for the accumulation of the vast majority of 24-nt This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. siRNAs (13–15), and it is involved in regulating the expression of 1073/pnas.1007972107/-/DCSupplemental. 12986–12991 | PNAS | July 20, 2010 | vol. 107 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.1007972107 Downloaded by guest on September 29, 2021 serve is carried out by mop1, a putative RDR, we performed nu- A NcoI NcoI clear run-on experiments using immature ears with B′ plants that Repeat 6 Repeat 7 were either wild type (WT) or homozygous for mop1-1. The results revealed that WT (B′)andB′ mop1-1 homozygous plants have very 84 77 79 81 83 85 A similar transcription levels from all the regions monitored (Fig. 1 7885 80 82 84 02 and B), demonstrating that the mop1-1 mutant did not reduce the b1TR transcription measured by nuclear run-on analysis. 8,000 B 2 b1TR Sequences Are Transcribed by a DNA-Dependent RNA Polymerase. 6,000 To test whether DNA-dependent RNA polymerases might be 4,000 2,000 contributing the majority of the transcription in nuclei, nuclear run- Counts/mm on reactions were performed with actinomycin D, an antibiotic that 0 83F 77R 84F 78R 83R 78F 79R 80F 82F 82R 02F 77F 84R 85F 79F 80R 81F 81R 02R forms stable complexes with DNA, blocking all DNA-templated 85R RNA synthesis. Results presented in Fig. 1C demonstrate that B’ B’ mop1-1 actinomycin D reduces transcription from the b1TR and down- 4% 23% 18% 32% 38% 24% 26% 26% 10% 12% 14% 25% 18% 20% 73% 27% 54% 39% stream sequences to levels similar to those of control genes tran- C 2 800 25,000 6,000 Ubiquitin2 18S 5,000 scribed by RNA polymerase II (Pol II; ) and Pol I ( ). 600 20,000 4,000 b1TR 15,000 These results demonstrate that transcription from the 400 3,000 10,000 2,000 and sequences immediately downstream is predominantly me- Counts/mm 200 5,000 1,000 diated by DNA-dependent RNA polymerases. Previous studies 0 0 0 have shown that b1 repeat transcription is not altered in a mu- 77F77R 78F 78R 85F 02F 02R Ubiquitin2 18S mop2 B’, Not treated B’ mop1-1, 50 g/mL actinomycin D tation of , which encodes the second largest subunit of a Pol B’ mop1-1, Not treated B’ mop1-1, 100 g/mL actinomycin D IV/Pol V-related complex (27). This suggests that Pol II might be the major polymerase contributing to b1 repeat transcription.
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