A novel initiator regulates expression of the nontissue-specific helix-loop-helix gene ME1

Human Müllerian-inhibiting substance promoter contains a functional TFII-I-binding initiator

Müllerian-inhibiting substance (MIS) plays an essential role in mammalian male sexual development; thus, it is important to determine how the tightly regulated expression of the MIS gene is transcriptionally controlled. Transcription of eukaryotic genes is dependent on regulatory elements in the enhancer and one or both distinct elements in the core promoter: the TATA box, and the initiator (Inr) element. Because the human MIS gene does not contain a consensus TATA and has not been reported to contain an Inr element, we hypothesized that the initiator region of the core promoter was essential for promoter activity. Transient transfection assays were conducted using an immortalized Embryonic Day 14.5 male rat urogenital ridge cell line (CH34) that expresses low levels of MIS. These studies revealed that promoter activity is dependent on the region around the start site (-6 to +10) but not on the nonconsensus TATA region. Electrophoretic mobility shift assays demonstrated that the human MIS initiator sequence forms a specific DNA-protein complex with CH34 cell nuclear extract, HeLa cell nuclear extract, and purified TFII-I. This complex could be blocked or supershifted by the addition of antibodies directed against TFII-I. These data suggest that the human MIS gene contains a functional initiator that is specifically recognized by TFII-I.

Selective conservation of an E-protein gene promoter during vertebrate evolution

The murine E-protein gene ME1 encodes a non-tissue-specific, helix-loop-helix transcription factor that is associated with morphological development. ME1 gene expression is regulated by a TATA-less promoter that contains multiple Sp1 consensus elements, E-boxes, and a novel transcription initiation site. In this study, we compared DNA homologous to the ME1 promoter from vertebrate species ranging from frog to human. A region of striking sequence similarity was identified in a region corresponding to the ME1 transcription initiation site (ME1 Inr). Within this region, a poly d(A) tract and a 9-bp inverted repeat (5'-GTCCGCCTG) were highly conserved in all species that were examined. Protein complexes that recognized these DNA elements were present among distant vertebrates (frog, chick, monkey and human), and were able to bend the ME1 Inr to a similar extent (approximately 60 degrees) as the previously described murine MBP alpha and MBP beta proteins. Collectively, these results suggest that an ME1 Inr-like element and its associated proteins functioned in an ancestral vertebrate more than 350 million years ago.

Transcription initiation from a poly(dA) tract

The mammalian ME1 gene encodes a non-tissue-specific, helix-loop-helix transcription factor that is enriched in morphogenetically active regions during development. Regulation of mouse ME1 gene expression is controlled by a novel initiator (ME1 Inr) that promotes transcription from the center of a 13 bp poly(dA) tract. We show here that the ME1 Inr autonomously directs initiation from the poly(dA) tract both in vitro and in vivo. This transcription was dependent upon two protein complexes; MBPalpha, which associated directly with the poly(dA) tract, and MBPbeta, which introduced an approximately 60 degrees bend immediately downstream of the poly(dA) tract. The MBPalpha and MBPbeta binding sites were strikingly conserved in homologous DNA from several mammalian species and the frog Xenopus laevis. These results suggest that the ME1 Inr constitutes a robust nucleation site that promotes transcription initiation in the absence of conventional promoter elements.

The initiator element and proximal upstream sequences affect transcriptional activity and start site selection in the amyloid beta-protein precursor promoter

The TATA-less human amyloid beta-protein precursor promoter contains an initiator element with the sequence CGTCA+1GTT. Primary transcriptional start sites were identified at positions +1 and -4. Deletion of the upstream activator elements APBbeta and APBalpha did not affect the selection of transcriptional start sites, although total transcriptional activity was reduced both in vitro and in vivo. Mutations within the initiator element shifted the transcriptional start sites and reduced transcriptional activity. Mutations between positions -6 and -35 changed the relative utilization of start sites +1 and -4 without affecting the total level of transcriptional activity. A 10-base pair deletion between position -40 and -31 increased in vitro transcriptional activity with a preeminent utilization of the start site at position -4. In contrast, a 20-base pair deletion between position -40 and -21 resulted in a reduction in transcriptional activity and in the primary utilization of the start site at position +1. Furthermore, transactivation by APBbeta and APBalpha was eliminated. DNase I footprinting provided evidence for the existence of two binding domains designated UE (position -12 to -30) and Inr (position +7 to -7). The positions of these binding domains are altered in mutations and deletions that affect transcriptional activity.

Characterization of a single-locus minisatellite DNA in Xenopus laevis

We have cloned a minisatellite tandem array (XTA) from Xenopus laevis that contains approximately 200 copies of the 20-bp repeat 5'-CCAACAGCCTGCCCATCCAT-3'. The XTA sequence is present only once per haploid genome and is polymorphic with respect to repeat number and location of flanking restriction endonuclease sites. Although the 20-bp repeat has not previously been described, flanking sequences suggest that it lies proximal to coding regions in the Xenopus genome.

Helix-loop-helix transcription factors regulate Id2 gene promoter activity

Id-like helix-loop-helix (HLH) transcription factors are involved in the regulation of proliferation and differentiation of several cell types. We isolated 5' regulatory region of mouse Id2 gene and demonstrated that it contains several E-box clusters. These E-boxes mediate stimulatory effects of basic-HLH (bHLH) transcription factors ME1, ME2, and NSCL1 on Id2 promoter activity. Co-expression of Id2 blocks the stimulatory effect of bHLH transcription factors which suggests the presence of feedback loops in Id2 transcriptional regulation. Overexpression of NSCL1 in F9 cells blocks the downregulation of Id2 gene expression during retinoic acid induced differentiation. Our data demonstrate that bHLH transcription factors regulate Id2 gene expression.