Identification of a protein that recognizes a distal negative regulatory element within the mouse mammary tumor virus long terminal repeat

Promoter complex in the central part of the mouse mammary tumor virus long terminal repeat

Unique among the retroviruses, mouse mammary tumor virus (MMTV) carries, in addition to the usual long terminal repeat (LTR) promoter, another promoter, P2, which is located in the central part of the proviral U3 sequence, within the LTR open reading frame (ORF). Using an in vitro reporter system based on a sensitive luciferase expression assay, we investigated the regulation of the P2 promoter in the context of the Mtv-2 and Mtv-8 genomes. Irrespective of the genomic source, the activity of the P2 promoter is regulated by a downstream-located enhancer and an upstream-located negative regulatory element (NRE), the activity of which overrides the activator. During this study, we unexpectedly detected another independent neighboring promoter that we called P3. The novel P3 promoter does not seem to be controlled by any NRE but is influenced by the same enhancer that modulates the P2 promoter. The respective transcription starts of the two promoters located in this tight cluster are only 61 bases apart. The transcripts originating from this promoter complex carry the same first intron, which is bound by canonical splice donor and splice acceptor sites located in the LTR. One novel doubly spliced transcript carrying a 459-nucleotide-long ORF was detected in several MMTV-carrying murine cells and could be successfully expressed in murine cells as a His-tagged fusion product. The novel viral protein, the function of which remains to be elucidated, has an apparent molecular mass of 20 kDa.

CDP binding to multiple sites in the mouse mammary tumor virus long terminal repeat suppresses basal and glucocorticoid-induced transcription

Mouse mammary tumor virus (MMTV) is transcribed at high levels in the lactating mammary gland to ensure transmission of virus from the milk of infected female mice to susceptible offspring. We previously have shown that the transcription factor CCAAT displacement protein (CDP) is expressed in high amounts in virgin mammary gland, yet DNA-binding activity for the MMTV long terminal repeat (LTR) disappears as mammary tissue differentiates during lactation. CDP is a repressor of MMTV expression and, therefore, MMTV expression is suppressed during early mammary gland development. In this study, we have shown using DNase I footprinting and electrophoretic mobility shift assays that there are at least five CDP-binding sites in the MMTV LTR upstream of those previously described in the promoter-proximal negative regulatory element (NRE). Single mutations in two of these upstream sites (+691 or +692 and +735 relative to the first base of the LTR) reduced CDP binding to the cognate sites and elevated reporter gene expression from the full-length MMTV LTR. Combination of a mutation in the promoter-distal NRE with a mutation in the proximal NRE gave approximately additive increases in LTR-reporter gene activity, suggesting that these binding sites act independently. Mutations in several different CDP-binding sites allowed elevation of reporter gene activity from the MMTV promoter in the absence and presence of glucocorticoids, hormones that contribute to high levels of MMTV transcription during lactation by activation of hormone receptor binding to the LTR. In addition, overexpression of CDP in transient-transfection assays suppressed both basal and glucocorticoid-induced LTR-mediated transcription in a dose-dependent manner. These data suggest that multiple CDP-binding sites contribute independently to regulate binding of positive factors, including glucocorticoid receptor, to the MMTV LTR during mammary gland development.

In vitro analysis of transcriptional repression of the mouse mammary tumor virus promoter

Transcription of the proviral DNA of mouse mammary tumor virus (MMTV) is induced by several classes of hormone-activated steroid receptor proteins. Basal promoter activity in the absence of receptor-mediated activation is selectively repressed by a distal negative regulatory element (dNRE) centered approximately 400 bp upstream of the transcription initiation site. An in vitro transcription system based on synthetic T-free cassette templates was developed to assess MMTV promoter activity, and dNRE-mediated repression was partially reconstituted with this system. Repression was observed with templates in which the dNRE was present in several sequence contexts. The activity of transcription preinitiation complexes formed in vitro in the presence of the dNRE could not be distinguished from that of complexes formed in its absence as assessed by the kinetics of transcript accumulation after addition of nucleoside triphosphates to preformed preinitiation complexes. dNRE-mediated repression in vitro appeared to be the result of decreased efficiency of assembly of functional transcription complexes on the MMTV promoter. However, repression could not be explained by inhibition of assembly of TATA-binding protein or transcription factor IIB into transcription complexes, as neither protein decreased the extent of repression when supplied in excess as a purified recombinant protein.

Discrete promoter elements affect specific properties of RNA polymerase II transcription complexes

The frequency of transcription initiation at specific RNA polymerase II promoters is, in many cases, related to the ability of the promoter to recruit the transcription machinery to a specific site. However, there may also be functional differences in the properties of assembled transcription complexes that are promoter-specific or regulator-dependent and affect their activity. Transcription complexes formed on variants of the adenovirus major late (AdML) promoter were found to differ in several ways. Mutations in the initiator element increased the sarkosyl sensitivity of the rate of elongation and decreased the rate of early steps in initiation as revealed by a sarkosyl challenge assay that exploited the resistance of RNA synthesis to high concentrations of sarkosyl after formation of one or two phospho-diester bonds. Similar, but clearly distinct, effects were also observed after deletion of the binding site for upstream stimulatory factor from the AdML promoter. In contrast, deletion of binding sites for nuclear factor 1 and Oct-1, as well as mutations in the recognition sequence for initiation site binding protein, were without apparent effect on transcription complexes on templates containing the mouse mammary tumor virus promoter.