Cellular factors involved in transcription and Tax-mediated trans-activation directed by the TGACGT motifs in human T-cell leukemia virus type I promoter.
Human T-cell leukemia virus type I (HTLV-I) encodes a 40-kDa nuclear protein, Tax, which stimulates transcription from three 21-base pair (bp) repeats in its U3 region. Tax trans-activation is mediated via cellular factors that interact with the TGACGT motifs in the 21-bp repeats. Gel mobility shift assay and UV cross-linking analysis show that two proteins of 52 and 46 kDa in size bind the 21-bp repeat specifically. Base substitutions in the TGACGT motif which abolished Tax trans-activation abrogated factor binding whereas the repeats containing mutations that did not affect Tax trans-activation supported factor binding as the wild-type repeat. The 52- and 46-kDa factors are present in human T-cell lines Jurkat and MT4 (HTLV-I transformed) and in HeLa cells but are undetectable in a human placental cell line JEG-3, which gave a reduced level of trans-activation. JEG-3 extracts contain a distinct DNA binding activity that shows analogous sequence requirements as the 52- and 46-kDa proteins in interacting with the various 21-bp repeats. c-Jun and CREB (cAMP-responsive element binding factor) can stimulate transcription from HTLV-I long terminal repeats in JEG-3 cells. At least two copies of the 21-bp repeats are required for optimal trans-activation by c-Jun and CREB. Most single point mutations in the TGACGT motif that abolished Tax trans-activation, however, did not affect c-Jun- or CREB-directed transcriptional enhancement. These data indicate that many transcription factors including c-Jun and CREB exert stimulatory effects on HTLV-I transcription although they do not directly respond to Tax. The 52- and 46-kDa cellular proteins most likely are involved directly in Tax-mediated trans-activation, and they are tentatively named Tax activation factors I and II, respectively.
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Department of Biochemistry, University of Nebraska Medical Center, College of Medicine, Omaha 68198-4525.
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Siores
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Gao
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D001483:Base Sequence; D002467:Cell Nucleus / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D016356:Gene Products, tax / Q000378:metabolism; D016355:Genes, pX; D006367:HeLa Cells / Q000378:metabolism; D015368:Human T-lymphotropic virus 1 / Q000235:genetics*; D006801:Humans; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes; D010957:Plasmids; D011401:Promoter Regions, Genetic; D015183:Restriction Mapping; D014158:Transcription, Genetic; D015533:Transcriptional Activation
D004268:DNA-Binding Proteins; D016356:Gene Products, tax; D015345:Oligonucleotide Probes