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Journal of Virology, November 2009, p. 11569-11580, Vol. 83, No. 22
0022-538X/09/$08.00+0     doi:10.1128/JVI.00742-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Loss of the Brm-Type SWI/SNF Chromatin Remodeling Complex Is a Strong Barrier to the Tat-Independent Transcriptional Elongation of Human Immunodeficiency Virus Type 1 Transcripts ^,

Taketoshi Mizutani,^1,3 Aya Ishizaka,¹ Mariko Tomizawa,² Takuya Okazaki,¹ Nobutake Yamamichi,¹ Ai Kawana-Tachikawa,² Aikichi Iwamoto,² and Hideo Iba^1*

Division of Host-Parasite Interaction, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo, Japan,¹ Division of Infectious Disease, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan,² RNA and Biofunctions, PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan³

Received 10 April 2009/ Accepted 26 August 2009

To elucidate the epigenetic regulation of Tat-independent human immunodeficiency virus (HIV) transcription following proviral integration, we constructed an HIV type 1 (HIV-1)-based replication-defective viral vector that expresses a reporter green fluorescent protein (GFP) product from its intact long terminal repeat (LTR). We transduced this construct into human tumor cell lines that were either deficient in or competent for the Brm-type SWI/SNF complex. One day after transduction, single cells that expressed GFP were sorted, and the GFP expression profiles originating from each of these clones were analyzed. Unlike clones of the SWI/SNF-competent cell line, which exhibited clear unimodal expression patterns in all cases, many clones originating from Brm-deficient cell lines either showed a broad-range distribution of GFP expression or were fully silenced. The resorting of GFP-negative populations of these isolated clones showed that GFP silencing is either reversible or irreversible depending upon the proviral integration sites. We further observed that even in these silenced clones, proviral gene transcription initiates to accumulate short transcripts of around 60 bases in length, but no elongation occurs. We found that this termination is caused by tightly closed nucleosome-1 (nuc-1) at the 5' LTR. Also, nuc-1 is remodeled by exogenous Brm in some integrants. From these results, we propose that Brm is required for the occasional transcriptional elongation of the HIV-1 provirus in the absence of Tat. Since the Brm-type SWI/SNF complex is expressed at marginal levels in resting CD4⁺ T cells and is drastically induced upon CD4⁺ T-cell activation, we speculate that it plays crucial roles in the early Tat-independent phase of HIV transcription in affected patients.

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* Corresponding author. Mailing address: Division of Host-Parasite Interaction, Department of Microbiology and Immunology, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan. Phone: 81-3-5449-5730. Fax: 81-3-5449-5449. E-mail: iba{at}ims.u-tokyo.ac.jp

Published ahead of print on 2 September 2009.

Supplemental material for this article may be found at http://jvi.asm.org/.

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Journal of Virology, November 2009, p. 11569-11580, Vol. 83, No. 22
0022-538X/09/$08.00+0     doi:10.1128/JVI.00742-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.