Research teams

Chromatin Architecture of uDNA
Histones are normally loaded onto newly synthesized DNA during DNA replication and DNA repair. During DNA replication “old” nucleosomes are distributed to both daughter branches and “new” nucleosomes are added to restore normal nucleosome density. To ensure an appropriate pattern of gene expression, epigenetic marks on the old chromatin are reestablished on newly synthesized DNA. Histone variants and histone modifications are the major regulator of genome functions. They confer variations in the higher-order chromatin at the nucleosome level, and regulate several DNA metabolisms.   The situation is different during retroviral infection. Indeed, upon HIV infection, viral genomic RNA is retrotranscribed into histone free vDNA which is imported into the nucleus and subsequently integrates into the host genome to ensure its maintenance and expression.  We addressed several challenging questions: How and when nucleosomes are loaded onto the vDNA? When are epigenetic marks placed on the nucleosomes of the vDNA? Why are unintegrated viral DNAs very poorly expressed and what are the cellular and/or viral factors involved?  We found that histones are loaded on HIV-1 DNA after its nuclear import and before its integration in the host genome. Nucleosome positioning analysis along the unintegrated and integrated viral genomes revealed major differences in nucleosome density and position. Indeed, in addition to the well-known nucleosomes Nuc0, Nuc1, and Nuc2 loaded on integrated HIV-1 DNA, we also found NucDHS, a nucleosome that covers the DNase hypersensitive site, in unintegrated viral DNA. The presence of NucDHS in the proximal region of the long terminal repeat (LTR) promoter was associated with the absence of RNAPII and of the active histone marks H3K4me3 and H3ac at the LTR. Conversely, analysis of integrated HIV-1 DNA showed a loss of NucDHS, loading of RNAPII, and enrichment in active histone marks within the LTR. We proposed that unintegrated HIV-1 DNA adopts a repressive chromatin structure that competes with the transcription machinery, leading to its silencing (Machida et al. 2020).

Host Factors Enforcing uDNA Silencing
To identify host factors responsible for uHIV-1 transcriptional repression, we used the proteomics of isolated chromatin segments method to reveal viral and host factors associated with unintegrated HIV-1 DNA involved in its silencing. Gene invalidation using siRNAs identified 46 factors as potential repressors of unintegrated HIV-1 DNA. Knockdown and knockout experiments revealed POLE3 as a transcriptional repressor of unintegrated HIV-1 DNA. POLE3 maintains unintegrated HIV-1 DNA in chromatin-repressed state, preventing RNAPII recruitment to the viral promoter. POLE3 and the recently identified host factors silencing unintegrated HIV-1 DNA CAF1 and SMC5/SMC6/SLF2 show specificity towards different forms of unintegrated HIV-1 DNA. Loss of POLE3 impaired HIV-1 replication, suggesting that repression of linear unintegrated HIV-1 DNA is important for optimal replication of the virus. POLE3 depletion reduces HIV-1 integration efficiency. Remarkably, POLE3, by maintaining a repressive chromatin structure of unintegrated HIV-1 DNA, ensures HIV-1 escape from innate immune sensing in primary CD4 T cell (Thenin-Houssier et al. 2023).

Chromatin-Mediated cGAS Inactivation
Innate immunity provides an important first line of defense against invading pathogens or harmful injury and relies on pattern recognition receptors (PRRs) expressed on the surface of or within immune cells to recognize pathogen-associated molecular patterns (PAMPs). Among known sensors, the cyclic GMP-AMP synthase (cGAS)–stimulator of interferon genes (STING) DNA pathway has emerged as a key innate immune pathway important for antiviral immunity. cGAS binds to double-stranded DNA and catalyzes the production of the second messenger 2’3’-cyclic GMP-AMP (cGAMP), a diffusible cyclic dinucleotide that activates the endoplasmic adaptor protein STING. Activated STING then functions as a platform to recruit and activate the kinase TBK1, which phosphorylates and activates the transcription factor IRF3, leading to induction of type I interferon (IFN) production. DNA sensors were initially proposed to be sequestered in the cytoplasm to avoid autoreactivity, with self-DNA protected by the nuclear membrane. However, recent studies have demonstrated that cGAS is abundant in the nucleus, where it is tightly tethered to chromatin by binding to the acidic patch formed by histones H2A and H2B in the nucleosome core. Importantly, when bound to nucleosomes, the catalytic domain of cGAS is buried, and cGAS is locked into a monomeric state, which prevents its dimerization and activation. Human immunodeficiency virus type 1 (HIV-1) is a retrovirus. Upon entry into the host cell, its RNA genome (vRNA) is transformed into double-stranded viral DNA (vDNA). vDNA is first loaded with histones and then integrated into the host genome, where it becomes indistinguishable from chromosomal DNA. By escaping from innate immune sensors, HIV-1 can efficiently replicate in target cells, including primary CD4+ T cells and macrophages. By protecting vRNA and vDNA from recognition by both RNA and DNA sensors, the viral core plays a key role in innate immune escape. However, how vDNA evades cGAS recognition after uncoating and before integration is unknown. In this study, we reveal a mechanism of HIV-1-mediated prevention of cGAS activation involving the uvDNA chromatin structure. cGAS is recruited to unintegrated HIV-1 DNA (uHIV-1 DNA) through its binding to viral DNA-associated nucleosomes, leading to its inactivation. Our work highlights the hijacking of chromatin-mediated cGAS inactivation as a mechanism used by HIV-1 for optimal evasion of innate immunity and suggests that other retroviruses and DNA viruses may exploit this mechanism for optimal evasion of cGAS sensing (Jahan et al. 2025).

In summary, several major discoveries were made:
1) revealing the mechanism and the biological significance of unintegrated HIV-1 transcriptional silencing.
2) The discovery of NucDHS loaded on the unintegrated HIV-1: implication in the establishment of latent provirus. This discovery calls for revisiting our approaches aiming at targeting the latent HIV-1 reservoir to achieve cure.
3) The interplay between chromatin and innate immune evasion mechanism evolved by HIV-1. The discovery of host factors regulating cGAS tethering to chromatin and a novel function for cGAS in chromatin organisation and function.

Title 3D genome structure and HIV integration site selection
Increasing evidence suggests that the 3D genome architecture is a key player in the regulation of genome function, particularly transcription. Indeed, by facilitating interactions between gene promoter and distal regulatory elements including enhancers, the 3D genome organization influences transcriptional regulation. HIV-1 gene expression has been shown to be dependent on integration site. More recently, the spatial organization of the eukaryotic nucleus was found to direct HIV integration into chromatin near the nuclear pore 3. We hypothesize that the spatial organization of the eukaryotic nucleus in which the viral pre-integration complex is found will dictate the integration site selection, impacting viral transcription. Elite Controllers (EC) are a small subset of people living with HIV (PLWH), able to spontaneously control HIV replication without ART. EC represents a natural model of HIV remission.  To determine the key genomic features associated with HIV ISs in CD4+ T cells isolated from EC (IS) and their impact on viral gene expression, we establish 3D genome organization and chromatin landscape in primary CD4 T cells from 4 HIV negative individuals.  We performed Chromosomal Conformation combined with high throughput sequencing (Hi-C) to identify global inter- and intra-chromosomal interactions and PCHi-C (promoter capture Hi-C) which identify the interactions between promoters and their regulatory elements within the genome. Hi-C data were used to rank genomic fragments according to their 3D interaction levels with distant sites. ISs identified from in vitro infected primary CD4 T cells isolated from uninfected individuals associated with productive infection (PIC) and from cART were enriched in highly connected genomic fragments. In contrast, we found that ISs from ECs and those associated with latent infection (LIC) were highly enriched in genomic regions with lowest connection to distant genomic sites. Next, we analyzed the interactions between ISs and enhancers/superenhancer using PCHi-C. Remarkably, while PIC and cART ISs are frequently connected to enhancers and superenhancers both in quiescent and active CD4 T cell, LIC and EC ISs are not. Taken together, these experiments identified two chromatin-associated features allowing to distinguish EC IS from those in non-EC.

Molecular basis of HBV persistence and HBV-induced liver cancer
Despite effective prevention against HBV infection, 250 million people worldwide are chronic HBV carriers, of whom 25% will die of liver cirrhosis or hepatocellular carcinoma (HCC). Current treatments for chronic hepatitis B (CHB) are inefficient to completely clear the virus and liver cancer is a lethal disease, thus representing an area of high unmet medical need. Viral persistence is due to the maintenance, in the nuclei of infected cells, of the viral nuclear DNA: the cccDNA that is not targeted by the antiviral treatment and to the impairment of both innate and adaptive immune responses that accompanies CHB infection.

An important emphasis was thus given on the study of the viral nuclear DNA (cccDNA) transcriptional regulation and its maintenance in the nucleus and on the mechanisms triggering tumorigenesis.  In order to better understand the mechanisms underlying HBV cccDNA establishment, transcription and maintenance we performed PiCh-MS using infected hepatoma cell lines and identified cellular proteins associated with cccDNA covering all the different stages of this molecule. We identified 198 proteins and among them the viral capsid (HBc) protein known to bind cccDNA as well as other host-factors known to bind cccDNA or viral proteins (such as HMGA1, SRSF10, SRPK1), thereby validating the experimental approach. We are currently investigating the impact of some of the most promising factors on HBV replication using loss of function approaches. Among the proteins identified in the PiCh we also identified H2AZ a cellular histone that we previously identified as a protein associated with the cccDNA in HBV-infected human primary hepatocytes as well as his two chaperones (SRCAP and ep400). Using silencing approaches in HepG2 NTCP cells infected with HBV, we showed that knocking down H2A.Z leads to a strong decrease of viral RNA expression as well as to a decrease in RNA polymerase II recruitment and H3K4me3 deposition.  Finally, knocking down SRCAP but not ep400 leads to a sharp decrease of viral RNA levels. Our results suggest that H2AZ and SRCAP are required for HBV transcription (Jay et al., manuscript in preparation). We are currently studying the impact of H2AZ recruitment on cccDNA formation. During the course of our work, we had to develop a method allowing the quantification of the HBV RNAs.   Indeed, because of the overlapping organization of HBV genes there is no straight forward method to date available to quantify individually the HBV RNAs which impedes our understanding of HBV biology. We thus set up a strategy based on Droplet Digital PCR (ddPCR) to specifically detect and precisely quantify each of the main HBV RNA species (HBx, PreS2/S, PreS1, SP1 and PreC/pgRNA) in infected cells. This method should contribute to the understanding of HBV biology and may help in the evaluation of the efficacy of HBV therapies (Sarica et al, submitted).

Molecular mechanisms underlying HBV-induced tumorigenesis are poorly understood and probably involve a combination of direct and indirect effects. Notably, the viral regulatory protein HBx is thought to be a co-factor in oncogenesis, but its role remains largely unknown. We recently characterized a subset of HCCs that replicate HBV (Halgand et al., J. hepatol. 2018) and using these tumors, we searched whether particular HBx variants are expressed in tumor tissues. We thus characterized HBx encoded by the full-length HBV DNA (cccDNA) in HCC and non-HCC liver and we showed that while they have retained their ability to activate cccDNA transcription, they tend to lose antiproliferative activity (Rivière et al., Antiviral Res. 2019). Our results raise the question of whether these tumor variants have acquired specific activities related to tumor development. Finally, while HBV integrations are found in 80 to 90% of HBV related HCC, their role in tumorigenesis are not fully understood. We thus investigated the consequence of HBV integration on the higher-order chromatin organization which are fundamental for precise regulation of gene expression. To address this question, we performed chromosome conformation capture (Hi-C) in HCC-derived cell lines containing integrated HBV. We identified intrachromosomal contacts between genomic regions containing HBV integrations and distant genomic regions which are enriched for cancer related genes. Moreover, silencing of integrated HBV DNA leads to the repression of contacted genes. These observations suggest that the integrated HBV genome can form new distal contacts with cellular genes, through DNA looping, that could deregulate cancer related genes expression (Dias et al., in preparation).

• Jay B. , Lopez O. , Gérossier L. , Sarica N. ,Pozo G., Gerbal-Chaloin S., Daujat M. , Hantz O. , Neuveut C. and Diogo Dias J. The histone variant H2A.Z and its chaperone SRCAP are required for RNA polymerase II to access HBV chromatin. 2025. bioRxiv2025 DOI 10.1101/2025.11.10.687638
• Sarica N., Meki I., Jay B., Lopez O., Petitjean G. and Neuveut C. (2025) . ddOTs: a multiplexed quantitative ddPCR approach for resolving overlapping hepatitis B virus transcripts to decipher cccDNA-driven transcription. BioRxiv doi: https://doi.org/10.1101/2025.08.27.672646.
• Jahan C, Bonnet-Madin L, Machida S, Sobhian B, Thenin-Houssier S, Benkirane M. Unintegrated HIV-1 DNA recruits cGAS via its histone-binding domain to escape innate immunity. Proc Natl Acad Sci U S A. 2025 Mar 18;122(11):e2424465122. doi: 10.1073/pnas.2424465122.
• Suzie Thenin-Houssier, Shinichi Machida, Cyprien Jahan, Lucie Bonnet-Madin, Scarlette Abbou, Heng-Chang Chen, Robel Tesfaye, Olivier Cuvier and Monsef Benkirane. POLE3 is a transcriptional repressor of unintegrated linear HIV-1 DNA required for efficient virus integration and escape from innate immune sensing. Science Advances. 2023 Nov 3;9(44):eadh3642. doi: 10.1126/sciadv.adh3642.
• Dias J. D., Sarica N. and Cournac A., Koszul R. and Neuveut C (2022). Crosstalk between Hepatitis B virus and the 3D genome structure. Viruses 2022, 14, 445. Doi : 10.3390/v14020445
• Machida S, Diogo Dias J, Benkirane M. Faithful to the Marseille tradition: Unique and intriguing-that’s how Marseillevirus packs its DNA. Mol Cell. 2022 Dec 1;82(23):4401-4402. doi: 10.1016/j.molcel.2022.11.001.
• Benkirane M. Condensates, the place to hide self-immunostimulatory RNA. Mol Cell. 2022 Oct 20;82(20):3754-3755. doi: 10.1016/j.molcel.2022.09.028.
• Shinichi Machida, David Depierre, Heng-Chang Chen, Suzie Thenin-Houssier, Gaël Petitjean, Cécile Doyen, Motoki Takaku, Olivier Cuvier and Monsef Benkirane. Exploring histone loading on unintegrated HIV DNA reveals a dynamic nucleosome positioning between unintegrated and integrated viral genome. PNAS. 117 (12), 6822-6830. doi: 10.1073/pnas.1913754117.
• Meertens L, Hafirassou ML, Couderc T, Bonnet-Madin L, Kril V, Kümmerer BM, Labeau A, Brugier A, Simon-Loriere E, Burlaud-Gaillard J, Doyen C, Pezzi L, Goupil T, Rafasse S, Vidalain PO, Legout AB, Gueneau L, Juntas-Morales R, Ben Yaou R, Bonne G, De Lamballerie X, Benkirane M, Roingeard P, Delaugerre C, Lecuit M and Amara A. FHL1 is a major host factor for chikungunya infection. Nature. 2019 Oct;574(7777):259-263.
• Rivière L., Quioc-Salomon B., Fallot G, Halgand B, Féray C., Buendia M.A. and Neuveut C. (2019). Hepatitis B virus replicating in hepatocellular carcinoma encodes HBx variants with preserved ability to antagonize restriction by Smc5/6. Antiviral Reserach 172: 104618.doi: 10.1016/j.antiviral. 2019.104618.
• Ringeard M, Marchand V, Decroly E, Motorin Y, Bennasser Yc. FTSJ3 is an RNA 2′-O-methyltransferase recruited by HIV to avoid innate immune sensing. Nature. 2019 Jan;565(7740):500-504. doi: 10.1038/s41586-018-0841-4. Epub 2019 Jan 9.
• Moreau P., Cournac A., Palumbo G. A., Marbouty M., Mortaza S., Thierry A., Cairo S., Lavigne M. , Koszul R. and Neuveut C. (2018). Tridimensional infiltration of DNA viruses into the host genome shows preferential contact with active chromatin. Nature Comm. Oct 15;9(1):4268. doi: 10.1038/s41467-018-06739-4.
• Mehdi Morchikh, Alexandra Cribier, Raoul Raffel, Sonia Amraoui, Julien Cau, Dany Severac, Emeric Dubois, Olivier Schwartz, Yamina Bennasser and Monsef Benkirane. HEXIM1 and NEAT1 lncRNA form a multi subunit complex that regulates DNA-mediated innate immune response. Mol Cell. 2017 Aug 3;67(3):387-399.
• Benjamin Descours, Gaël Petitjean, José-Luis López-Zaragoza, Timothée Bruel, Raoul Raffel, Christina Psomas, Jacques Reynes, Christine Lacabaratz, Yves Levy, Olivier Schwartz, Jean Daniel Lelievre and Monsef Benkirane. Identification of a marker of CD4 T cell HIV reservoir harboring replication-competent provirus. Nature 2017 Mar 23;543(7646):564-567. Cited 107 times. See also Descours et al. Nature. 2018 Sep;561(7723).
• Brégnard C, Guerra J, Déjardin S, Passalacqua F, Benkirane M*, Laguette N*. Upregulated LINE-1 Activity in the Fanconi Anemia Cancer Susceptibility Syndrome Leads to Spontaneous Pro-inflammatory Cytokine Production. EBioMedicine. 2016 Jun;8:184-94.
• Rivière L., Gerossier L., Ducroux A., Dion S., Deng Q., Michel M.L. , Buendia M.A., Hantz O. and Neuveut C. (2015). HBx relieves chromatin-mediated transcriptional repression of hepatitis B viral cccDNA involving SETDB1 histone methyltransferase. J. Hepatol. doi: 10.1016/j.jhep.2015.06.023
• Bernd Stadelmayer, Gaël Micas, Adrien Gamot, Pascal Martin, Nathalie Malirat, Slavik Koval, Raoul Raffel, Bijan Sobhian, Dany Severac, Stéphanie Rialle, Hugues Parrinello, Olivier Cuvier and Monsef Benkirane. Integrator complex regulates NELF-mediated RNA Polymerase II pause/release and processivity at coding genes. Nat Commun. 2014 Nov 20;5:5531. doi: 10.1038/ncomms6531.
• Nadine Laguette, Christelle Brégnard, Pauline Hue, Jihane Basbous, Ahmad Yatim, Marion Larroque, Frank Kirchhoff, Angelos Constantinou, Bijan Sobhian and Monsef Benkirane. Premature activation of the SLX4 complex by Vpr promotes G2/M arrest and escape from innate immune sensing. Cell. 2014 Jan 16;156(1-2):134-45.
• Ducroux A, Benhenda S, Rivière L, Semmes OJ, Benkirane M, Neuveut C. (2014). The Tudor domain protein Spindlin1 is involved in intrinsic antiviral defense against incoming hepatitis B Virus and herpes simplex virus type 1. PloS Pathog. 10 : e1004343.
• Ruth Clifford*, Tania Louis*, Sam Ackroyd, Adam Burns, Pauline Robbe, Adele T Timbs, Glen Colopy, Helene Dreau, Francois Sigaux, Jean Gabriel Judde, Margalida Rotger, Amalio Telenti, Yea-Li Lin, Philippe Pasero, Jonathan Maelfait, Shirley J Henderson, Mark T Ross, David Bentley, Peter Hillmen, Jenny C Taylor, Jan Rehwinkel, Samantha JL Knight, Yanick J Crow, Monsef Benkirane§, Anna Schuh§. SAMHD1 is recurrently mutated in chronic lymphocytic leukaemia and engages in specific interactions upon DNA damage. Blood 2014, 123 (7), 1021-1031.
• Alexandra Cribier, Benjamin Descours, Ana Luiza Chaves Valadão, Nadine Laguette, and Monsef Benkirane. 2013. Phosphorylation of SAMHD1 by CyclinA2/CDK1 regulates its restriction activity towards HIV-1. Cell Rep 2013 Apr 25;3(4):1036-43.
• Benhenda S., Ducroux A.,Rivière L., Sobhian B., Ward M., Dion S., Hantz O., Protzer U., Michel M.L., Benkirane M., Semmes O.J., Marie-Annick Buendia M.A. and Neuveut C. (2013). The PRMT1 methyltransferase is a binding partner of HBx and a negative regulator of hepatitis B virus transcription. J. Virol. : 87: 4360-4370.
• Benjamin Descours, Alexandra Cribier, Christine Chable-Bessia, Gillian Rice, Yanick Crow, Ahmad Yatim, Nadine Laguette and Monsef Benkirane. SAMHD1 restricts HIV-1 replication in quiescent CD4+ T-cells. Retrovirology 2012 Oct 23;9:87.
• Ahmad Yatim, Clarisse Benne, Bijan Sobhian, Sabine Laurent-Chabalier, Olivier Deas, Jean-Gabriel Judde, Jean Daniel Lelievre, Yves Levy and Monsef Benkirane. NOTCH1 nuclear interactome reveals key regulators of its transcriptional activity and oncogenic function. Mol Cell 2012 Sep 25.doi: 10.1016/j.molcel.2012.08.022. [Epub ahead of print].
• Alexandre Wagschal, Emilie Rousset, Poornima Basavarajaiah, Xavier Contreras, Alexander Harwig, Sabine Laurent-Chabalier, Mirai Nakamura, Xin Chen, Ke Zhang, Oussama Meziane, Frédéric Boyer, Hugues Parrinello, Ben Berkhout, Christophe Terzian, Monsef Benkirane and Rosemary Kiernan. Microprocessor, Setx, Xrn2 and Rrp6 Co-Operate to Induce RNAPII Pausing and Premature Termination in Mammalian Cells. Cell 2012. Sep 14;150(6):1147-57.
• Laguette N, Rahm N, Sobhian B, Chable-Bessia C, Munch J, Snoeck J, Sauter D, Switzer WM, Heneine W, Kirchhoff F, Delsuc F, Telenti A, Benkirane M. 2012. Evolutionary and Functional Analyses of the Interaction between the Myeloid Restriction Factor SAMHD1 and the Lentiviral Vpx Protein. Cell Host Microbe 2012 Feb 16;11(2):205-17.
• Cougot D., Allemand E., Rivière L., Benhenda S., Duroure K., Levillayer L., Muchard C., Buendia M.A. and Neuveut C. (2012). Inhibition of PP1 phosphatase activity by HBx: a new mechanism for the activation of hepatitis B virus transcription. Sci. Signal. 2012. Janvier 3 ; 5(205 :RA1)
• Laguette N, Sobhian B, Casartelli N, Ringeard M, Chable-Bessia C, Segeral E, Yatim A, Emiliani S, Schwartz O, Benkirane M. 2011. SAMHD1 is the dendritic- and myeloid-cell-specific HIV-1 restriction factor counteracted by Vpx. Nature 474: 654-7.
• Bennasser Y, Chable-Bessia C, Triboulet R, Gibbings D, Gwizdek C, Dargemont C, Kremer EJ, Voinnet O, Benkirane M. 2011. Competition for XPO5 binding between Dicer mRNA, pre-miRNA and viral RNA regulates human Dicer levels. Nat Struct Mol Biol 18: 323-7.
• Sobhian B, Laguette N, Yatim A, Nakamura M, Levy Y, Kiernan R, Benkirane M. 2010. HIV-1 Tat assembles a multifunctional transcription elongation complex and stably associates with the 7SK snRNP. Mol Cell 38: 439-51.
• Triboulet R, Mari B, Lin YL, Chable-Bessia C, Bennasser Y, Lebrigand K, Cardinaud B, Maurin T, Barbry P, Baillat V, Reynes J, Corbeau P, Jeang KT, Benkirane M. 2007. Suppression of microRNA-silencing pathway by HIV-1 during virus replication. Science 315: 1579-82.
• Du Chene I, Basyuk E, Lin YL, Triboulet R, Knezevich A, Chable-Bessia C, Mettling C, Baillat V, Reynes J, Corbeau P, Bertrand E, Marcello A, Emiliani S, Kiernan R, Benkirane M. 2007. Suv39H1 and HP1gamma are responsible for chromatin-mediated HIV-1 transcriptional silencing and post-integration latency. Embo J 26: 424-35.
• Cougot D.,Wu Y, Cairo S. , Caramel J. , Renard C.A. , Levy L., Buendia M.A. and Neuveut C. (2007). The hepatitis B virus X protein functionally interacts with CBP/p300 in the regulation of CREB-mediated transcription. J. Biol. Chem 282: 4277-87.
• Lévy L; Wei Y., Labalette C., Wu Y., Renard C.A., Buendia M.A. and Neuveut C. (2004).Acetylation of ß-catenin by p300 regulates ß-catenin-Tcf4 interaction. Mol Cell. Biol. 24 :3404-14.
• Bres V, Kiernan RE, Linares LK, Chable-Bessia C, Plechakova O, Treand C, Emiliani S, Peloponese JM, Jeang KT, Coux O, Scheffner M, Benkirane M. 2003. A non-proteolytic role for ubiquitin in Tat-mediated transactivation of the HIV-1 promoter. Nat Cell Biol 5: 754-61.