Tissue-specific expression from a compound TATA-dependent and TATA-independent promoter

Heterogeneity in the 5' untranslated region of mouse cytochrome cT mRNAs leads to altered translational status of the mRNAs

Previous studies have shown that the differential regulation of mouse somatic cytochrome c (cyt cS) and testicular cytochrome c (cyt cT) during spermatogenesis is accompanied by changes in mRNA length [Hake et al. (1990) Development, 110, 249-257]. When analyzed by polysomal gradient sedimentation, cytochrome cT sediments in two broad size classes: non-polysomal mRNAs are about 0.6 to 0.75 kb and polysomal mRNAs range from 0.7 to 0.9 kb. Both classes of mRNAs shorten to about 0.5 kb following deadenylation. Oligonucleotide-directed cleavage of the cytochrome cT RNAs by RNase H reveals that the size heterogeneity of cytochrome cT mRNAs resides in the 5' untranslated regions (UTRs). Ribonuclease protection assays reveal that multiple cytochrome cT mRNAs are transcribed from six different transcriptional start sites spanning a region of 59 nucleotides in the 5'UTR from +1 to +59. Transcripts derived from the first and second transcriptional initiation sites are not loaded onto polysomes as efficiently as those transcripts initiated from the other start sites. Each of the longer mRNAs has an upstream open reading frame, which starts at +8 and ends at +136 in the 5'UTR of the cytochrome cT transcript. Computer analysis suggests that the lengthened 5'UTR sequences allow additional hairpin structures to be formed. Since the upstream open reading frame and the additional stem loop structure are absent in the 5' UTRs of the cytochrome cT mRNAs initiated from the four downstream start sites, we suggest that these sequences in the two longest cytochrome cT transcripts hinder their loading onto polysomes.

TATA-dependent enhancer stimulation of promoter activity in mice is developmentally acquired

Herpes simplex virus (HSV) thymidine kinase (tk) promoter activity depends on four transcription factor binding sites, one of which is a TATA box sequence, and the presence of either a cis-acting enhancer sequence or a transactivator protein. Studies presented here show that this TATA box was required for promoter activity only after cells began to differentiate and then only when promoter activity was stimulated by either an enhancer or a transactivator. When the HSV tk promoter was utilized by mouse embryos from the one-cell to eight-cell stage of development or by undifferentiated mouse embryonic stem cells, disruption of the HSV tk TATA box by site-specific mutations did not reduce promoter activity. This was true even when HSV tk promoter activity was stimulated strongly by either the embryo-responsive polyomavirus F101 enhancer or its natural transactivator, the HSV ICP4 gene product. However, stimulated expression was dependent on a distal Sp1 DNA binding site. Similarly, disruption of the TATA box did not reduce tk promoter activity in primary mouse embryonic fibroblasts or in immortalized 3T3 mouse fibroblasts; in fact, promoter activity was increased up to 2.6-fold. However, in these differentiated cells, stimulation of the HSV tk promoter by either the F101 enhancer or ICP4 protein required the TATA box. HSV tk promoter activity also was dependent on its TATA box in the mouse oocyte, a terminally differentiated cell with an endogenous transactivating activity. These results reveal that the need for a TATA box is developmentally acquired and depends on at least two parameters: the differentiated state of the cell and stimulation of the promoter by either an enhancer or a transactivator.

TATA-dependent enhancer stimulation of promoter activity in mice is developmentally acquired

Herpes simplex virus (HSV) thymidine kinase (tk) promoter activity depends on four transcription factor binding sites, one of which is a TATA box sequence, and the presence of either a cis-acting enhancer sequence or a transactivator protein. Studies presented here show that this TATA box was required for promoter activity only after cells began to differentiate and then only when promoter activity was stimulated by either an enhancer or a transactivator. When the HSV tk promoter was utilized by mouse embryos from the one-cell to eight-cell stage of development or by undifferentiated mouse embryonic stem cells, disruption of the HSV tk TATA box by site-specific mutations did not reduce promoter activity. This was true even when HSV tk promoter activity was stimulated strongly by either the embryo-responsive polyomavirus F101 enhancer or its natural transactivator, the HSV ICP4 gene product. However, stimulated expression was dependent on a distal Sp1 DNA binding site. Similarly, disruption of the TATA box did not reduce tk promoter activity in primary mouse embryonic fibroblasts or in immortalized 3T3 mouse fibroblasts; in fact, promoter activity was increased up to 2.6-fold. However, in these differentiated cells, stimulation of the HSV tk promoter by either the F101 enhancer or ICP4 protein required the TATA box. HSV tk promoter activity also was dependent on its TATA box in the mouse oocyte, a terminally differentiated cell with an endogenous transactivating activity. These results reveal that the need for a TATA box is developmentally acquired and depends on at least two parameters: the differentiated state of the cell and stimulation of the promoter by either an enhancer or a transactivator.

Cathepsin D gene is controlled by a mixed promoter, and estrogens stimulate only TATA-dependent transcription in breast cancer cells

The cathepsin D (cath-D) gene, coding for a ubiquitous lysosomal aspartyl protease, is overexpressed in aggressive human breast cancers, and its transcription is induced by estrogens in hormone-responsive breast cancer cells. We have determined the structure and function of the proximal 5' upstream region of the human cath-D gene from MCF7 cells. We show that the promoter has a compound structure with features of both housekeeping genes (high G+C content and potential transcription factor Sp1 sites) and regulated genes (TATAA sequence). By RNase protection assay, we show that transcription is initiated at five major transcription sites (TSSI to -V) spanning 52 base pairs. In hormone-responsive breast cancer cells, estradiol increased by 6- to 10-fold the level of RNAs initiated at TSSI, which is located about 28 base pairs downstream from the TATA box. The specific regulation by estradiol of transcription starting at site I exclusively was confirmed by primer extension. Moreover, the same estradiol effect was observed in the ZR75-1 cell line and in MDA-MB231 estrogen-resistant breast cancer cells stably transfected with the estrogen receptor. Site-directed mutagenesis indicated that the TATA box is essential for initiation of cath-D gene transcription at TSSI. In breast cancer biopsy samples, high levels of TATA-dependent transcription were correlated with overexpression of cath-D mRNA. We conclude that cath-D behaves, depending on the conditions, as a housekeeping gene with multiple start sites or as a hormone-regulated gene that can be controlled from its TATA box.

A second ferritin L subunit is encoded by an intronless gene in the mouse

Multiple homologous sequences for the ferritin L subunit are present in mammalian genomes, but so far, only one expressed gene has been described. Here we report the isolation of a cDNA from a mouse bone marrow library, corresponding to an isoform of the mouse ferritin L subunit. This new subunit, that we named Lg, differs from the L subunit of ten amino acids. Specific amplification of mouse genomic DNA using the polymerase chain reaction (PCR) confirmed the presence of this Lg sequence in the mouse genome but also suggested that it must be encoded by an intronless gene. Using a series of different Lg-specific oligonucleotides as probes, we subsequently isolated a genomic clone containing an uninterrupted sequence, identical to the Lg cDNA. This Lg gene lacks introns and does not contain the 28 base pairs (bp) conserved motif usually present at the 5' end of most ferritin mRNAs, which confers translational regulation by iron. When transiently transfected into K562 cells, this Lg genomic clone is actively transcribed, suggesting that, although it possesses the characteristics of a processed pseudogene, it is likely to correspond to the gene encoding this new ferritin subunit.

The herpes simplex virus type 1 thymidine kinase is expressed in the testes of transgenic mice under the control of a cryptic promoter

We reported previously that the herpes simplex virus type 1 (HSV-1) thymidine kinase reporter gene (tk) was expressed in the testes of transgenic mice when coupled to the promoter of a liver-specific mouse major urinary protein (MUP) gene. Here we show that HSV-1 tk is also expressed in the testis when coupled to a MUP pseudogene promoter, to a truncated MUP promoter that is not active in the liver, and to the promoter of the bovine thyroglobulin gene. Furthermore, HSV-1 tk itself was expressed in the testis, although its normal expression had been disabled by removing an upstream regulator of transcription. In every case, the same multiple transcripts were observed, with their 5' ends located downstream of the normal HSV-1 tk translation initiation codon. We conclude that the transcription of HSV-1 tk in the testis is directed by a cryptic TATA box-independent promoter located in the coding region of the gene. The longest HSV-1 thymidine kinase (TK) polypeptides synthesized in the testis were shorter than full-length TK and probably result from translational initiation at Met46 and Met60, the second and third ATG codons of the tk reading frame. Male mice of most transgenic lines were sterile, and the severity of the lesion in spermatogenesis was directly related to the level of TK expression. In the most highly expressing lines, sperm counts were low and morphologically defective sperm were common. In other sterile lines, TK was expressed at a lower level and sperm counts were normal but sperm motility was greatly reduced. Lines with the lowest levels of HSV-1 TK expression were fertile. HSV-1 TK was expressed in germ line cells, mainly in the haploid spermatids. However, low-level HSV-1 TK activity was found in the testis before the first germ cells entered meiosis, showing that if expression is confined to the germ cells, it also occurs in spermatogonia.

A ubiquitous enhancer shared by two promoters in the human aldolase A gene

The human aldolase A gene is transcribed from three different promoters, which are all clustered within a 1.6 kbp DNA domain. Two of these, PN and PH, are ubiquitous and seem to be co-regulated in most tissues while the third one, PM, is specific to adult skeletal muscle. We investigated the sequences involved in the ubiquitous activity of the PN and PH promoters of the human aldolase A gene. Deletion analysis, performed by transient expression assays of chloramphenicol acetyltransferase reporter genes in human HepG2 hepatoma cells, indicated that PH activity results from the interaction of an upstream activating region with two distinct core promoters. The upstream activating region was able to stimulate transcription from the HSV tk promoter as efficiently as the SV40 enhancer in all cell types tested. It appears, therefore, to be a strong ubiquitous enhancer. DNAsel footprinting revealed protections covering sequences scattered along the enhancer, including Sp1 and AP1 motifs. Importantly, we found that this enhancer was also necessary to activity of the other ubiquitous promoter of the aldolase A gene, PN. These studies demonstrate that expression of the human aldolase A gene is mediated by a complex interplay of enhancer and promoter elements.

The herpes simplex virus type 1 thymidine kinase is expressed in the testes of transgenic mice under the control of a cryptic promoter

We reported previously that the herpes simplex virus type 1 (HSV-1) thymidine kinase reporter gene (tk) was expressed in the testes of transgenic mice when coupled to the promoter of a liver-specific mouse major urinary protein (MUP) gene. Here we show that HSV-1 tk is also expressed in the testis when coupled to a MUP pseudogene promoter, to a truncated MUP promoter that is not active in the liver, and to the promoter of the bovine thyroglobulin gene. Furthermore, HSV-1 tk itself was expressed in the testis, although its normal expression had been disabled by removing an upstream regulator of transcription. In every case, the same multiple transcripts were observed, with their 5' ends located downstream of the normal HSV-1 tk translation initiation codon. We conclude that the transcription of HSV-1 tk in the testis is directed by a cryptic TATA box-independent promoter located in the coding region of the gene. The longest HSV-1 thymidine kinase (TK) polypeptides synthesized in the testis were shorter than full-length TK and probably result from translational initiation at Met46 and Met60, the second and third ATG codons of the tk reading frame. Male mice of most transgenic lines were sterile, and the severity of the lesion in spermatogenesis was directly related to the level of TK expression. In the most highly expressing lines, sperm counts were low and morphologically defective sperm were common. In other sterile lines, TK was expressed at a lower level and sperm counts were normal but sperm motility was greatly reduced. Lines with the lowest levels of HSV-1 TK expression were fertile. HSV-1 TK was expressed in germ line cells, mainly in the haploid spermatids. However, low-level HSV-1 TK activity was found in the testis before the first germ cells entered meiosis, showing that if expression is confined to the germ cells, it also occurs in spermatogonia.