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Sp1, Sp3 (SPR‐2) and Sp4 (SPR‐1) are human sequence‐specific DNA binding proteins with very similar structural features. In this report, we have analyzed Sp3 in direct comparison with Sp1. We have raised antibodies against both Sp1 and Sp3, and show that Sp3 protein, like Sp1, is expressed in various cell lines. Co‐transfection experiments in different mammalian cell lines reveal that in contrast to Sp1 and Sp4, Sp3 is not able to activate several Sp1 responsive promoters. In addition, Sp3 also fails to activate reporter constructs in Drosophila SL2 cells lacking endogenous Sp factors. Instead, we find that Sp3 represses Sp1‐mediated activation in a linear dose‐dependent manner. A mutant of Sp3 lacking the DNA binding domain does not affect activation by Sp1, suggesting that the inhibition is most likely due to the competition with Sp1 for their common binding sites. To determine if any structurally similar domain of Sp3 is able to replace partially homologous domains of Sp1, we have generated chimeric proteins and tested their activation characteristics in gene transfer experiments. It appears that neither the glutamine‐rich domains A and B nor the D domain of Sp1 can be replaced by the homologous regions of Sp3. Our results suggest that Sp3 is an inhibitory member of the Sp family.

The molecular mechanisms by which ovarian hormones stimulate growth of breast tumors are unclear. It has been reported previously that estrogens activate the signal-transducing Src/p21 ras /Erk pathway in human breast cancer cells via an interaction of estrogen receptor (ER) with c-Src. We now show that progestins stimulate human breast cancer T47D cell proliferation and induce a similar rapid and transient activation of the pathway which, surprisingly, is blocked not only by anti-progestins but also by anti-estrogens. In Cos-7 cells transfected with the B isoform of progesterone receptor (PR B ), progestin activation of the MAP kinase pathway depends on co-transfection of ER. A transcriptionally inactive PR B mutant also activates the signaling pathway, demonstrating that this activity is independent of transcriptional effects. PR B does not interact with c-Src but associates via the N-terminal 168 amino acids with ER. This association is required for the signaling pathway activation by progestins. We propose that ER transmits to the Src/p21 ras /Erk pathway signals received from the agonist-activated PR B . These findings reveal a hitherto unrecognized cross-talk between ovarian hormones which could be crucial for their growth-promoting effects on cancer cells.

Previous analyses of the uteroglobin gene promoter revealed a GT1 box which is also found in the SV40 enhancer. The GT1 element in the context of the uteroglobin promoter is active in Ishikawa cells, a human endometrial cell line, but not in HeLa cells. Here we report the cloning by recognition site screening of two factors (SPR-1 and SPR-2) which bind to this GT1 motif. SPR-1 and SPR-2 are homologues of the transcription factor Sp1. All three proteins are closely related members of a gene family encoding proteins with very similar structural features. Like Sp1, SPR-1 and SPR-2 contain glutamine and serine/threonine rich amino acid stretches. Most significantly, the DNA binding domains of all three proteins are highly conserved and they recognize GT as well as GC boxes identically. SPR-2 mRNA is expressed ubiquitously, whereas SPR-1 transcripts are abundant in the brain but barely detectable in other organs. The possible function of these factors for the activity of the uteroglobin promoter is discussed.

Chromatin structure influences transcription, but its role in subsequent RNA processing is unclear. Here we present analyses of high-throughput data that imply a relationship between nucleosome positioning and exon definition. First, we have found stable nucleosome occupancy within human and Caenorhabditis elegans exons that is stronger in exons with weak splice sites. Conversely, we have found that pseudoexons--intronic sequences that are not included in mRNAs but are flanked by strong splice sites--show nucleosome depletion. Second, the ratio between nucleosome occupancy within and upstream from the exons correlates with exon-inclusion levels. Third, nucleosomes are positioned central to exons rather than proximal to splice sites. These exonic nucleosomal patterns are also observed in non-expressed genes, suggesting that nucleosome marking of exons exists in the absence of transcription. Our analysis provides a framework that contributes to the understanding of splicing on the basis of chromatin architecture.

Deletion experiments have defined two stretches of DNA (genetic elements), lying close to the promoter for a human gene for metallothionein, that separately mediate the induction of the gene by heavy metal ions, particularly cadmium, and by glucocorticoid hormones. The element responsible for induction by cadmium is duplicated, yet a single copy is fully functional; the element responsible for induction by glucocorticoid hormones is coincident with the DNA-binding site for the glucocorticoid hormone receptor.