Konesky Kasey L, Nyborg Jennifer K, Laybourn Paul J
Department of Biochemistry and Molecular Biology, Colorado State University, 1870 Campus Delivery, Fort Collins, CO 80523-1870, USA.
J Virol. 2006 Nov;80(21):10542-53. doi: 10.1128/JVI.00631-06. Epub 2006 Aug 30.
Upon infection of human T-cell leukemia virus type 1 (HTLV-1), the provirus is integrated into the host cell genome and subsequently packaged into chromatin that contains histone H1. Consequently, transcriptional activation of the virus requires overcoming the environment of chromatin and H1. To efficiently activate transcription, HTLV-1 requires the virally encoded protein Tax and cellular transcription factor CREB. Together Tax and CREB interact with three cis-acting promoter elements called viral cyclic-AMP response elements (vCREs). Binding of Tax and CREB to the vCREs promotes association of p300/CBP into the complex and leads to transcriptional activation. Therefore, to fully understand the mechanism of Tax transactivation, it is necessary to examine transcriptional activation from chromatin assembled with H1. Using a DNA template harboring the complete HTLV-1 promoter sequence and a highly defined recombinant assembly system, we demonstrate proper incorporation of histone H1 into chromatin. Addition of H1 to the chromatin template reduces HTLV-1 transcriptional activation through a novel mechanism. Specifically, H1 does not inhibit CREB or Tax binding to the vCREs or p300 recruitment to the promoter. Rather, H1 directly targets p300 acetyltransferase activity. Interestingly, in determining the mechanism of H1 repression, we have discovered a previously undefined function of Tax, overcoming the repressive effects of H1-chromatin. Tax specifically abrogates the H1 repression of p300 enzymatic activity in a manner independent of p300 recruitment and without displacement of H1 from the promoter.
人类1型T细胞白血病病毒（HTLV-1）感染后，前病毒整合到宿主细胞基因组中，随后被包装到含有组蛋白H1的染色质中。因此，病毒的转录激活需要克服染色质和H1的环境。为了有效激活转录，HTLV-1需要病毒编码的蛋白Tax和细胞转录因子CREB。Tax和CREB共同与三个顺式作用启动子元件相互作用，这些元件称为病毒环磷酸腺苷反应元件（vCREs）。Tax和CREB与vCREs的结合促进p300/CBP与复合物的结合，并导致转录激活。因此，为了全面了解Tax反式激活的机制，有必要研究从与H1组装的染色质进行的转录激活。使用携带完整HTLV-1启动子序列的DNA模板和高度明确的重组组装系统，我们证明了组蛋白H1正确整合到染色质中。向染色质模板中添加H1通过一种新机制降低了HTLV-1的转录激活。具体而言，H1不抑制CREB或Tax与vCREs的结合，也不抑制p300募集到启动子。相反，H1直接靶向p300乙酰转移酶活性。有趣的是，在确定H1抑制机制时，我们发现了Tax以前未定义的功能，即克服H1-染色质的抑制作用。Tax以独立于p300募集的方式且不将H1从启动子上置换的方式特异性地消除了H1对p300酶活性的抑制。
