Giam C Z, Xu Y L
Department of Biochemistry, University of Nebraska Medical Center, College of Medicine, Omaha 68105.
J Biol Chem. 1989 Sep 15;264(26):15236-41.
Human T-cell leukemia virus type I (HTLV-I) is the etiological agent of adult T-cell leukemia. The 3' end of HTLV-I proviral DNA encodes the synthesis of two regulatory proteins, tax and rex. The 40-kDa tax protein is a nuclear protein which positively stimulates transcription from the U3 region of the viral long terminal repeat sequence. Three 21-base pair sequences in the U3 region have been found to serve as the cis-element for tax-mediated trans-activation. We now report that the tax protein can trans-activate HTLV-I LTR in the absence of de novo cellular protein synthesis. Saturated mutagenesis of the 21-base pair repeat sequence showed that specific mutations clustered in sequences homologous to the cAMP responsive element (TGACGTCA) abolish trans-activation by tax. Furthermore, although the TGACGTCN element is nearly palindromic, the mutations that abolish trans-activation are localized exclusively in the 5' 6 bases, suggesting the orientation of this element may play a role in transcription. That the purified tax protein does not bind the 21-base pair repeats or nonspecific DNA lends further support to the notion that tax protein does not directly interact with the 21-base pair repeats to activate transcription. Instead, tax most likely acts via cellular transcriptional factor(s) to bring about trans-activation.
人类T细胞白血病病毒I型（HTLV-I）是成人T细胞白血病的病原体。HTLV-I前病毒DNA的3'端编码两种调节蛋白tax和rex的合成。40 kDa的tax蛋白是一种核蛋白，可正向刺激病毒长末端重复序列U3区域的转录。已发现U3区域中的三个21碱基对序列作为tax介导的反式激活的顺式元件。我们现在报告，在没有新生细胞蛋白合成的情况下，tax蛋白可以反式激活HTLV-I长末端重复序列（LTR）。对21碱基对重复序列进行饱和诱变表明，特定突变聚集在与cAMP反应元件（TGACGTCA）同源的序列中，可消除tax的反式激活作用。此外，尽管TGACGTCN元件几乎是回文结构，但消除反式激活作用的突变仅定位在5'端的6个碱基中，这表明该元件的方向可能在转录中起作用。纯化的tax蛋白不与21碱基对重复序列或非特异性DNA结合，这进一步支持了tax蛋白不直接与21碱基对重复序列相互作用以激活转录的观点。相反，tax最有可能通过细胞转录因子起作用以实现反式激活。
