Although many genes are regulated by estrogen, very few have been shown to directly bind the estrogen receptor complex. Therefore, transcriptional cascades probably occur in which the estrogen receptor directly binds to a target gene that encodes another transcription factor that subsequently regulates additional genes. Through the use of a differential display assay, a transcription factor has been identified that may be involved in estrogen transcriptional cascades. This report demonstrates that transcription factor ␦EF1 is induced eightfold by estrogen in the chick oviduct. Furthermore, the regulation by estrogen occurs at the transcriptional level and is likely to be a direct effect of the estrogen receptor complex, as it does not require concomitant protein synthesis. A putative binding site was identified in the 5-flanking region of the chick ovalbumin gene identifying it as a possible target gene for regulation by ␦EF1. Characterization of this binding site revealed that ␦EF1 binds to and regulates the chick ovalbumin gene. Thus, a novel regulatory cascade that is triggered by estrogen has been defined.

We have identified a class of proteins that bind single-stranded telomeric DNA Telomeres, the DNA-protein complexes at the ends of linear chromosomes, stabilize and protect the chromosomal termini (reviewed in refs. 1 and 2). Chromosomes often appear highly organized within the nucleus, with the telomeres clustered at the nuclear periphery (3). Telomere-associated proteins are thought to participate in telomere replication, the protection of telomeric DNA from degradation, and the associations between telomeres and other nuclear structures. Telomeric DNA from many organisms is composed of short repeat sequences that include multiple guanines and few, if any, cytosines on the strand that forms the 3' end of the chromosomal DNA (G-strand) (reviewed in ref. 4).In Saccharomyces cerevisiae, Raplp binds double-stranded (ds) telomeric DNA in vitro (5-7) and in vivo (8-10) and is required for telomere length control (11) and telomere position effect (TPE) (12), the transcriptional silencing of genes placed close to the chromosome ends (13). Raplp also binds to single-stranded (ss) yeast telomeric G-strand DNA, albeit with a much lower affinity than it binds to ds telomeric DNA (14).In pachytene spreads, Raplp localizes primarily to telomeres (15), and, in interphase, Raplp localizes to a small number of perinuclear spots (15) that colocalize with telomericThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.DNA (H. Scherthan, T. Laroche, and S. Gasser, personal communication). Mutations in genes that alter telomere length (11) and TPE (12) also change the localization of Rapip from punctate and perinuclear to diffuse and nuclear (11). Taken together, these results suggest that wild-type, interphase telomeres associate with Raplp and with one another primarily at the nuclear periphery (3,11,15). While Raplp binds ds telomeric DNA, chromosomal termini are characterized by short ssDNA overhangs (16,17).A number of proteins that bind ss G-strand telomeric DNA have been characterized. In hypotrichous ciliates, telomeric protein-DNA complexes are resistant to high salt and DNA in the complexes is protected from exonucleolytic degradation (18,19). The ciliate telomere-binding proteins bind 3' overhang sequences and share a high degree of similarity (20-23). Like the ciliate telomere-binding proteins, a Xenopus egg protein, X-TEF, binds specifically to vertebrate 3' overhang telomere sequences in vitro (24). Several vertebrate proteins that bind in vitro to ss G-strand telomeric DNA have also been identified. These include heteronuclear ribonucleoproteins (hnRNPs) A2/B1, Al, D, and E (25-27) as well as lamins and vimentin (28). Because the hnRNPs appear to have a much higher affinity for r(UUAGGG)n RNA substrates than for the cognate telomeric DNA sequence d(TTAGGG)n, the role, if any, of hnRNPs at telomeres is not clear (25,27).Gbplp is a Chlamydomonas reinhardti...