Cloning of GT box-binding proteins: a novel Sp1 multigene family regulating T-cell receptor gene expression

A Drosophila homologue of human Sp1 is a head-specific segmentation gene

Segmentation in Drosophila is based on a cascade of hierarchical gene interactions initiated by maternally deposited morphogens that define the spatially restricted domains of gap gene expression at blastoderm (reviewed in ref. 1). Although segmentation of the embryonic head is morphologically obscured, the repeated patterns of expression of the segment polarity genes reflect the formation of seven head segments; two of these depend on the segmentation and homeotic genes used in the trunk, whereas the others form as a result of the activity of the head-specific genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd). The genes ems and otd encode homeodomain proteins, suggesting that they may function as transcription factors. They are expressed in overlapping stripes in the early embryonic head of Drosophila, and their vertebrate homologues, otx and emx, are expressed in overlapping domains in the anterior central nervous system of the mouse embryo. We show here that btd is expressed in a stripe covering the head analgen of the segments affected in btd lack-of-function mutants and that btd encodes a zinc-finger-type transcription factor with sequence and functional similarity to the prototype mammalian transcription factor Sp1 (ref. 9). When expressed in the spatial pattern of btd, a transgene providing Sp1 activity can support development of the mandibular segment in the head of btd mutant embryos. A ubiquitous transcription factor from humans can therefore replace an essential component of the genetic circuitry required to specify the development of a particular head segment in the fly.

Identification and analysis of the chicken c-mil promoter: possible involvement of Sp1- and Ets-related proteins

We have determined the exon organization in the 5' region of the chicken c-mil gene and identified its promoter. A 0.44-kb fragment containing the 5' terminus of the c-mil gene showed strong promoter activity when placed upstream of the bacterial chloramphenicol acetyltransferase gene and transfected into chicken embryo fibroblasts (CEF). By primer extension analysis, multiple transcriptional start sites were detected within the promoter region. Nucleotide sequence analysis revealed that the c-mil promoter had a high G + C content (71.8%) and contained multiple GC box-like sequences, but no TATA or CAAT boxes. Deletion analysis of 5' upstream sequences showed that the minimal region required for maximal promoter activity in CEF resides in the 99 bp located immediately upstream of the major initiation site. This region contains two putative Sp1 binding sites and one PU box/PEA3 motif, defined as a recognition element for members of the Ets gene family. These sequences bound proteins present in nuclear extracts of CEF as well as in vitro synthesized Ets-related proteins, suggesting that the binding of Sp1 or related proteins and of Ets-related proteins within the promoter is important for modulation of the mil gene.

Determinants of the brain-specific expression of the rat aldolase C gene: ex vivo and in vivo analysis

A 115-bp promoter fragment of the aldolase C gene is sufficient for conferring neural cell specificity on a reporter gene, in cultured PC12 cells and in transgenic mice. In vitro DNase I protection experiments detected two footprints on the promoter, termed boxes A/A', and B. The 5' A/A' box contains overlapping Sp1 and Krox20/Krox24 binding sites; it binds Sp1 in fibroblasts (box A') and a different complex in brain (box A). Any deletion or mutation of this box that impairs protein recognition also suppresses promoter activity. The replacement of box A/A' by a Sp1 consensus binding site results in the loss of the brain specificity of expression in transgenic mice. Further 3', box B is composed of a 5' direct repeat and a 3' GC box consisting of overlapping Sp1 and Krox20/Krox24 binding sites. Mutation of the direct repeat subregion appears to be more deleterious for the promoter activity than mutation of the G+C-rich subregion.

Rat ciliary neurotrophic factor (CNTF): gene structure and regulation of mRNA levels in glial cell cultures

The structure of the rat ciliary neurotrophic factor (CNTF) gene and the regulation of CNTF mRNA levels in cultured glial cells were investigated. The rat mRNA is encoded by a simple two-exon transcription unit. Sequence analysis of the region upstream of the transcription start-site did not reveal a typical TATA-box consensus sequence. Low levels of CNTF mRNA were detected in cultured Schwann cells, and CNTF mRNA was not increased by a variety of treatments. Three-week-old astrocyte-enriched cell cultures from new-born rat brain contained easily detectable CNTF mRNA. In astrocyte-enriched cultures, upregulation of CNTF mRNA levels was observed after treatment with IFN-gamma. CNTF mRNA levels were down-regulated in these cells by treatments that elevate intracellular cyclic AMP and by members of the fibroblast growth factor (FGF) family. The implications of these results for potential in vivo functions of CNTF are discussed.

Activation of the human T-cell leukemia virus type I enhancer is mediated by binding sites for Elf-1 and the pets factor

Infection with human T-cell leukemia virus type I (HTLV-I) is associated with adult T-cell lymphoma/leukemia. This disease occurs in only a small minority of people infected with HTLV-I and manifests itself many years after infection. Therefore, it appears that a fine balance exists between HTLV-I and the host T-cell factors with which it interacts. HTLV-I encodes a transactivating protein, Tax, which activates viral transcription via cellular mechanisms which are incompletely understood. As viral gene expression is negligible during latency, it is doubtful that Tax controls the initial transition to the replicative state. Tax-independent cellular factors which control HTLV-I transcription, and presumably latency, have received little study. Recently, the product of the chicken proto-oncogene ets-1 has been shown to bind to the HTLV-I enhancer and modestly activate transcription in certain cell types (S. C. Gitlin, R. Bosselut, A. Gégonne, J. Ghysdael, and J. N. Brady, J. Virol. 65:5513-5523, 1991). However, the functional significance of the ets-binding site in the intact enhancer has not previously been shown. We now demonstrate that site-specific mutation of the purine-rich ets-binding site significantly diminishes inducible enhancer function, but not Tax response, in the human Jurkat T-cell line. Similarly, mutation of the peri-ets (pets) site, not previously noted in the HTLV-I enhancer, markedly inhibits inducible enhancer function but not Tax response. Further, we show that the predominant protein binding the purine-rich HTLV-I enhancer element in human T cells is not ets-1 but Elf-1, a member of the ets family which is very similar to the Drosophila morphogen E74. Regulation of HTLV-I through Elf-1/pets enhancer motifs resembles that seen with human immunodeficiency virus type 2 (D. M. Markovitz, M. Smith, J. Hilfinger, M. C. Hannibal, B. Petryniak, and G. J. Nabel, J. Virol. 66:5479-5484, 1992; J. M. Leiden, C.-W. Wang, B. Petryniak, M. Smith, D. M. Markovitz, G. J. Nabel, and C. B. Thompson, J. Virol. 66:5890-5897, 1992), another human pathogenic retrovirus with a relatively long incubation period.

The US-1 element from the gene encoding rat poly(ADP-ribose) polymerase binds the transcription factor Sp1

By comparing the upstream DNA sequence of the rat and human genes encoding poly(ADP-ribose) polymerase (PARP), we have defined a 16-bp conserved region and designated it as US-1 for 'upstream sequence 1'. This element is homologous to the recently described binding site for the transcription factor Sp1 in the promoter sequence of the mouse p12 gene which encodes a protease inhibitor. Analyses in gel mobility shift assays revealed that a nuclear protein, produced by all tissue-culture cells tested, specifically binds the US-1 element. The pattern of shifted DNA protein complexes obtained was strikingly similar to that for Sp1, which is supported by the positive displacement of these complexes by an oligomer containing the Sp1 binding site in gel shift competition experiments. Replacement of the Sp1 binding site from the basal promoter of the mouse p12 gene by the rPARP US-1 element did not result in any significant variations in the level of expression of the chloramphenicol acetyltransferase (CAT) reporter gene upon transient transfection of tissue-culture cells. However, when point mutations are introduced in the US-1 element in a similar substitution experiment, a significant reduction in CAT gene expression could be observed. These data are consistent with Sp1 interacting with the US1 element. Results from DNase I footprinting experiments clearly indicated that purified Sp1 not only binds to the US-1 element but also to four other closely located cis-acting sites scattered in the promoter of the rat PARP gene, therefore suggesting that Sp1 is likely to modulate strongly the expression of that gene in different tissues.

Regulation of the beta-globin locus

Transcription of the human beta-globin gene cluster depends upon upstream regulatory sequences, which are collectively termed the locus control region. Recent studies have provided new insights into how the individual genes of the cluster are regulated through development. The crux of transcriptional activation is how the locus control region communicates with the gene-proximal regulatory elements.