Activation of the human epsilon- and beta-globin promoters by SV40 T antigen.

We have studied the effect of the SV40 T antigen on expression from human globin promoters fused to the bacterial chloramphenicol acetyltransferase (CAT) gene and compared its effect with the SV40 enhancer and the adenovirus E1A protein. We have observed that expression of p epsilon GLCAT and p beta GLCAT (the epsilon-globin or beta-globin promoter linked to the CAT gene) was significantly stimulated when cotransfected with a cloned T antigen plasmid into CV-1 cells, indicating that trans-activation of the globin promoters was mediated by SV40 T antigen. Transfection of the p beta GLCAT-SV (p beta GLCAT containing the SV40 enhancer element) into CV-1 cells resulted in a 50-60-fold increase in CAT activity as compared to p beta GLCAT (no enhancer). However, cotransfection of the p beta GLCAT-SV with the cloned T antigen resulted in an additional increase of CAT expression, which suggests that T antigen and the SV40 enhancer activate globin gene expression independently. We found that T antigen but not E1A could further stimulate the expression of an enhancer-containing plasmid in CV-1 cells; whereas E1A but not T antigen could further stimulate p epsilon GLCAT expression in COS-1 cells which constitutively express the SV40 T antigen. These results suggest that T antigen and E1A also act independently. Deletion analysis showed that the minimum sequence required for a detectable level of stimulation of the epsilon-globin promoter by T antigen is 177 bp 5' to the cap site, suggesting that the target sequences for response to T antigen do not reside in the canonical 100 bp promoter region, but rather reside in sequences further upstream, and therefore the cellular factors interacting with T antigen are not the TATA or CAT box binding proteins, but the proteins interacting with upstream regulatory sequences.     

Activation of an enhancerless gene by chromosomal integration.

Expression of enhancerless (E-) and enhancer-containing (E+) genes that are chromosomally integrated was examined. An E- plasmid (pE-cat) containing a chloramphenicol acetyltransferase (cat) gene linked to the simian virus 40 (SV40) early promoter or its E+ counterpart plasmid (pE+-cat) containing the SV40 enhancer was cotransfected into thymidine kinase (TK)-deficient L cells with a cloned tk gene. A number of TK+ transformants were isolated, and expression of the cointegrated cat gene in these cell lines was quantitatively determined by the assay of CAT activity. The results indicated unexpectedly that the E- cat gene was as actively expressed as the E+ cat gene. Analysis of CAT mRNA by primer extension indicated that the E- cat gene, as well as the E+ cat gene, was transcribed from the "native" initiation site contained in the SV40 early promoter region. The active expression of the E- cat gene was maintained in secondary TK+ transformants that arose by transfection with genomic DNA from the primary transformant. These results suggest that expression of the integrated E- cat gene is activated by endogenous enhancer elements.     

The differentiation-dependent inhibition of SV40 early promoter/enhancer activity in 3T3-L1 cells is mediated through promoter and not enhancer sequences

Using a plasmid bearing chloramphenicol acetyltransferase (CAT) gene controlled by Simian virus 40 (SV40) early promoter/enhancer complex (pA0cat), we analyzed functional enhancer motifs in 3T3-L1 fibroblast and adipocyte cells. Deletion mutant series of pA0 at the enhancer complex showed that gene expression both in fibroblast and adipocyte cells was dependent on a similar set of enhancer motifs. When pA0 was introduced into 3T3-L1 fibroblasts and the cells were induced to differentiate into adipocytes, CAT activity expressed in fibroblasts was suppressed. Experiments with the deletion mutants at the enhancer complex showed that the suppression was not related to any enhancer motif, and CAT activity was observed with a plasmid having only the promoter sequence. When pA0cat was co-transfected with excess of promoter sequence, the suppression in adipocytes was counteracted. This suggested that negativetrans-acting factors of the promoter sequence were responsible for the suppression in adipocytes.Abbreviations CAT chloramphenicol acetyltransferase - CAT the gene encoding CAT - SV40 Simian virus 40 - Asc-P ascorbic acid phosphate     

The promoter of the endo A cytokeratin gene is activated by a 3' downstream enhancer.

Mouse cytokeratin EndoA is an intermediate filament subunit of the type II cytokeratin class which initiates expression in trophectoderm cells of blastocyst during embryogenesis. To identify the regulatory elements of the endo A gene, we constructed a series of CAT expression vectors and transfected them into PYS-2 cells. We found an enhancer element locating 1 kb downstream from the endo A gene which acts on both the endo A and SV40 promoters. This enhancer consists of six direct repeated sequences with homology to the PEA3 motif in polyoma virus alpha enhancer core. In undifferentiated F9 embryonal carcinoma cells, expression of the construct containing the enhancer was not detected. These results indicate that one of the regulatory mechanisms of endo A gene expression is the 3' downstream enhancer.