Previous work has shown that the 68-kDa immediate-early protein of herpes simplex virus type 1 (HSV-1), also known as ICP22, is involved in the control of viral gene expression, although the precise mechanism remains to be elucidated. In order to study the function(s) of this protein, we constructed expression vectors containing the coding sequence of the ICP22 gene placed under the control of the SV40 or HCMV promoter. After cell transfection, ICP22 synthesis was studied by immunoblotting, using a specific antiserum. In transient expression experiments in COS cells in which the ICP22 vector was under the control of the SV40 promoter, we found that ICP22 was able to inhibit chloramphenicol acetyltransferase (CAT) expression under the control of either the alpha 22 (IE4) promoter or other immediate-early promoters, such as alpha 4 (IE3), alpha 0 (IE1), and alpha 27 (IE2). CAT expression under the control of the alpha 4 (IE3) promoter was inhibited in these cells by expression of ICP22 under the control of the HCMV promoter; it was also inhibited in RAT-1 cells by ICP22 expressed under the control of the SV40 or HCMV promoter. In contrast, CAT expression directed by the SV40 or HCMV promoters was only weakly or not inhibited by the ICP22 vectors. We also constructed an expression vector for UL13, a gene whose product is implicated in the phosphorylation of ICP22. Although CAT expression under the control of the alpha 4 (IE3) promoter was also negatively regulated by the UL13 gene product, the effects of the ICP22 (directed by the SV40 or HCMV promoter) and UL13 vectors were not synergistic; furthermore, at a particular molar ratio of the two vectors, inhibition of CAT activity was partially reversed. The results in the present work suggest that ICP22 can negatively regulate the expression of immediate-early viral genes and that its phosphorylation by UL13 protein kinase might be involved in the modulation of its function.
Transcription of both Xenopus thyroid hormone receptor (TR) genes, xTR alpha and -beta, is strongly up-regulated by their own ligand T3 during natural or T3-induced metamorphosis of tadpoles and in some Xenopus cell lines. To explain this autoinduction, we analyzed the sequence of 1.6 kilobases of xTR beta promoter for putative T3-responsive elements. Two direct repeat +4 AGGTCA hexamer motifs (DR+4), an imperfect distal (-793/-778) and a perfect proximal (-5/11) site, a DR+1 site, and some possible half-sites were located in the 1.6-kilobase promoter. Transfection of Xenopus XTC-2 cells (which express xTR alpha and -beta) and XL-2 cells (which predominantly express TR alpha) with chloramphenicol acetyltransferase reporter constructs of deletion mutants and promoter fragments showed that the distal and proximal DR+4 sites responded to T3, although other flanking sequences may also play a role. The thyroid hormone-responsive element half-site present as DR+1 in the up-stream sequence at -1260/-950, when cloned in front of a heterologous promoter, functions independently. T3 enhanced transcription from the two DR+4-containing fragments when present together by only 2- to 3-fold due to a high basal activity. Overexpression of unliganded xTR alpha and xTR beta in XTC-2 cells repressed basal activity, which was then enhanced 7- to 4-fold by T3, respectively; with XL-2 cells cotransfected with xTR beta, T3 inducibility increased to 16-fold. Electrophoretic mobility shift assays with recombinant Xenopus TR alpha, TR beta, retinoid-X receptor-alpha (RXR alpha) and RXR gamma proteins showed that TR-RXR heterodimers, but not TR or RXR monomers or homodimers, strongly bound the natural and synthetic distal and proximal DR+4 elements in a ligand-independent manner. TR/RXR heterodimers exhibited the highest binding affinity for a 28-mer oligonucleotide probe for the -5/11 proximal DR+4 site, with only slight binding to DR+1 (retinoid-X-responsive element-like) site. The xTR beta promoter binding to XTC-2 cell nuclear extract suggested the in vivo relevance of the findings with recombinant TR/RXR heterodimers. It is concluded that xTR alpha and -beta proteins are capable of regulating the expression of xTR beta gene, which can explain its autoinduction seen during T3-induced metamorphosis.
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