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Previous studies identified two regions in the U3 region of a molecular clone of simian immunodeficiency virus, SIVmac142, that are important to transcriptional activity under conditions of induction as well as basal-level expression (B. Renjifo, N. A. Speck, S. Winandy, N. Hopkins, and Y. Li, J. Virol. 64:3130-3134, 1990). One region includes the NF-kappa B binding site, while the other lies just 5' of this site between nucleotides -162 and -114 (the -162 to -114 region). The fact that the NF-kappa B site mutation attenuated transcriptional activity in uninduced T cells and fibroblasts where activated NF-kappa B would not be present suggested that a factor(s) other than NF-kappa B could be acting through this site. In this study, we have identified a factor which binds to a cis element overlapping the NF-kappa B site. This factor, which we call simian factor 3 (SF3), would play a role in regulation under conditions of basal level expression, whereas under conditions of induction, NF-kappa B would act via this region. SF3 may also bind to an element in the -162 to -114 region. In addition, we have identified two other factors that bind the -162 to -114 region. One, which we designated SF1, is a ubiquitous basal factor, and the other, SF2, is a T-cell-predominant phorbol myristate acetate-inducible factor. Through identification of nuclear factors that interact with the U3 region of the SIVmac142 long terminal repeat, we can gain insight into how this virus is transcriptionally regulated under conditions of basal-level expression as well as conditions of T-cell activation.

A series of 5' deletions and a point mutation in the binding site for nuclear factor kappa B were introduced into the U3 region of a molecular clone of simian immunodeficiency virus from macaques (SIVmac142). The transcriptional activity of the mutated U3 regions was analyzed by transient chloramphenicol acetyltransferase assays. Two distinct regions in U3 appeared to contain important cis-acting sequences for transcriptional activity. Mutation of the single nuclear factor kappa B site in the SIVmac142 U3 region attenuated transcription in Rat-1 fibroblasts and Jurkat T cells. A second cis-acting element was localized to sequences between -162 and -114 in U3; deletion of long terminal repeat sequences up to -114 significantly attenuated transcriptional activity in Rat-1 cells. Furthermore, sequences between -162 and -114 contributed to inducibility of transcription by 1,3-phorbol myristate acetate in Jurkat T cells. Deletion of long terminal repeat sequences to -114, in addition to mutation of the nuclear factor kappa B site, was necessary to attenuate the response to 1,3-phorbol myristate acetate completely. A negative regulatory element analogous to that identified in the U3 region from the human immunodeficiency virus was not found in the U3 region from SIVmac142.