Simian virus 40 small-t does not transactivate RNA polymerase II promoters in virus infections

Abstract

Transcriptional stimulatory properties of virus-encoded transactivators appear to be critical for viral gene expression and may be linked to cellular transformation in certain cases. Recently, the simian virus 40 (SV40) 17-kDa small-t antigen was shown to stimulate transcription of polymerase II and III genes in transient transfection assays. In experiments performed in our laboratory, two of the polymerase II promoters of the adenovirus genome, namely, the ElI-early and EIII promoters, were transactivated by the small-t antigen in transient transfection assays. To further elucidate the mechanism of this transactivation, we examined the ability of small-t to transactivate the adenovirus type 5 EIl-early and EIII promoters in CV-1 cells under conditions in which the small-t gene or the reporter genes were introduced into the cells through transfection and other routes. In one approach, we used established CV-1 cell lines which constitutively express the small-t gene, and study of the EII-early promoter was afforded by infection of an EIA-negative adenovirus type 5 variant. For the second approach, a recombinant adenovirus was constructed in which small-t was expressed from a replication origin-negative SV40 early promoter in the EIA region of an adenovirus vector (Ad-SV-t). The effect of small-t on adenovirus EII-early and EIII promoter expression was studied in coinfection or single-infection experiments. In both cases, transcription of the adenovirus early promoters was not stimulated by small-t. These and other results indicate that transactivation of polymerase II promoters by small-t occurs only when the target gene is in a transiently transfected state. Thus, small-t-mediated transactivation of polymerase II promoters is dependent on the type of assay system used and may be mechanistically different from that of the widely studied EIA.

Mutations which affect the inhibition of protein phosphatase 2A by simian virus 40 small-t antigen in vitro decrease viral transformation

“…Effect of the 97-103 region mutations on transactivation. Another known function of small-t antigen is its ability to transactivate the adenovirus E2 promoter (12,13,19). To determine whether the Cys 97-103 region played a role in this activity, the mutations were cloned into plasmid pw2t for cotransfection with a plasmid in which chloramphenicol acetyltransferase was expressed under the control of the adenovirus E2 promoter as in earlier studies (9, 19).…”

“…It has also been shown that small-t can transactivate polymerase II-and polymerase 111-directed transcription from a variety of promoters when cotransfected with reporter constructs (13). The relevance of this observation to infection has been called into question by the failure of small-t expressed from an adenovirus vector to activate transcription of the adenovirus E2 promoter (19). Small-t transactivation appears to require E2F-binding sites in the E2 promoter (12).…”

Common and unique features of T antigens encoded by the polyomavirus group

“…The unique carboxyl termini of the small t antigens are created by differential splicing of a common precursor mRNA. The SV40 small t antigen has been reported to transactivate the adenovirus E2 promoter in some contexts (17) but not in others (29). In addition, the small t antigens from PyV and SV40 have been shown to form a complex with the catalytic subunit and one regulatory subunit of protein phosphatase 2A (21,43).…”