Comparative Analysis of the Regulation of Expression and Structures of Two Evolutionarily Divergent Genes for Δ1-Pyrroline-5-Carboxylate Synthetase from Tomato1

We isolated two tomato (Lycopersicon esculentum) cDNA clones, tomPRO1 and tomPRO2, specifying Δ1-pyrroline-5-carboxylate synthetase (P5CS), the first enzyme of proline (Pro) biosynthesis. tomPRO1 is unusual because it resembles prokaryotic polycistronic operons (M.G. García-Ríos, T. Fujita, P.C. LaRosa, R.D. Locy, J.M. Clithero, R.A. Bressan, L.N. Csonka [1997] Proc Natl Acad Sci USA 94: 8249–8254), whereas tomPRO2 encodes a full-length P5CS. We analyzed the accumulation of Pro and the tomPRO1 and tomPRO2 messages in response to NaCl stress and developmental signals. Treatment with 200 mm NaCl resulted in a >60-fold increase in Pro levels in roots and leaves. However, there was a <3-fold increase in the accumulation of the tomPRO2 message and no detectable induction in the level of the tomPRO1 message in response to NaCl stress. Although pollen contained approximately 100-fold higher levels of Pro than other plant tissues, there was no detectable increase in the level of either message in pollen. We conclude that transcriptional regulation of these genes for P5CS is probably not important for the osmotic or pollen-specific regulation of Pro synthesis in tomato. Using restriction fragment-length polymorphism mapping, we determined the locations of tomPRO1 and tomPRO2 loci in the tomato nuclear genome. Sequence comparison suggested that tomPRO1 is similar to prokaryotic P5CS loci, whereas tomPRO2 is closely related to other eukaryotic P5CS genes.

Developmental and Light Regulation of Desacetoxyvindoline 4-Hydroxylase in Catharanthus roseus (L.) G. Don.1 : Evidence of a Multilevel Regulatory Mechanism

The expression of desacetoxyvindoline 4-hydroxylase (D4H), which catalyzes the second to the last reaction in vindoline biosynthesis in Catharanthus roseus, appears to be under complex, multilevel developmental and light regulation. Developmental studies with etiolated and light-treated seedlings suggested that although light had variable effects on the levels of d4h transcripts, those of D4H protein and enzyme activity could be increased, depending on seedling development, up to 9- and 8-fold, respectively, compared with etiolated seedlings. However, light treatment of etiolated seedlings could stop and reverse the decline of d4h transcripts at later stages of seedling development. Repeated exposure of seedlings to light was also required to maintain the full spectrum of enzyme activity observed during seedling development. Further studies showed that a photoreversible phytochrome appeared to be involved in the activation of D4H, since red-light treatment of etiolated seedlings increased the detectable levels of d4h transcripts, D4H protein, and D4H enzyme activity, whereas far-red-light treatment completely reversed this process. Additional studies also confirmed that different major isoforms of D4H protein exist in etiolated (isoelectric point, 4.7) and light-grown (isoelectric point, 4.6) seedlings, suggesting that a component of the light-mediated activation of D4H may involve an undetermined posttranslational modification. The biological reasons for this complex control of vindoline biosynthesis may be related to the need to produce structures that could sequester away from cellular activities the cytotoxic vinblastine and vincristine dimers that are derived partially from vindoline.

Direct Evidence for Rapid Degradation of Bacillus thuringiensis Toxin mRNA as a Cause of Poor Expression in Plants1

It is well established that the expression of Bacillus thuringiensis (B.t.) toxin genes in higher plants is severely limited at the mRNA level, but the cause remains controversial. Elucidating whether mRNA accumulation is limited transcriptionally or posttranscriptionally could contribute to effective gene design as well as provide insights about endogenous plant gene-expression mechanisms. To resolve this controversy, we compared the expression of an A/U-rich wild-type cryIA(c) gene and a G/C-rich synthetic cryIA(c) B.t.-toxin gene under the control of identical 5′ and 3′ flanking sequences. Transcriptional activities of the genes were equal as determined by nuclear run-on transcription assays. In contrast, mRNA half-life measurements demonstrated directly that the wild-type transcript was markedly less stable than that encoded by the synthetic gene. Sequences that limit mRNA accumulation were located at more than one site within the coding region, and some appeared to be recognized in Arabidopsis but not in tobacco (Nicotiana tabacum). These results support previous observations that some A/U-rich sequences can contribute to mRNA instability in plants. Our studies further indicate that some of these sequences may be differentially recognized in tobacco cells and Arabidopsis.

Expression of Tobacco Carbonic Anhydrase in the C4 Dicot Flaveria bidentis Leads to Increased Leakiness of the Bundle Sheath and a Defective CO2-Concentrating Mechanism

Flaveria bidentis (L.) Kuntze, a C4 dicot, was genetically transformed with a construct encoding the mature form of tobacco (Nicotiana tabacum L.) carbonic anhydrase (CA) under the control of a strong constitutive promoter. Expression of the tobacco CA was detected in transformant whole-leaf and bundle-sheath cell (bsc) extracts by immunoblot analysis. Whole-leaf extracts from two CA-transformed lines demonstrated 10% to 50% more CA activity on a ribulose-1,5-bisphosphate carboxylase/oxygenase-site basis than the extracts from transformed, nonexpressing control plants, whereas 3 to 5 times more activity was measured in CA transformant bsc extracts. This increased CA activity resulted in plants with moderately reduced rates of CO2 assimilation (A) and an appreciable increase in C isotope discrimination compared with the controls. With increasing O2 concentrations up to 40% (v/v), a greater inhibition of A was found for transformants than for wild-type plants; however, the quantum yield of photosystem II did not differ appreciably between these two groups over the O2 levels tested. The quantum yield of photosystem II-to-A ratio suggested that at higher O2 concentrations, the transformants had increased rates of photorespiration. Thus, the expression of active tobacco CA in the cytosol of F. bidentis bsc and mesophyll cells perturbed the C4 CO2-concentrating mechanism by increasing the permeability of the bsc to inorganic C and, thereby, decreasing the availability of CO2 for photosynthetic assimilation by ribulose-1,5-bisphosphate carboxylase/oxygenase.

Developmental Expression and Substrate Specificities of Alfalfa Caffeic Acid 3-O-Methyltransferase and Caffeoyl Coenzyme A 3-O-Methyltransferase in Relation to Lignification1

The biosynthesis of monolignols can potentially occur via two parallel pathways involving free acids or their coenzyme A (CoA) esters. Caffeic acid 3-O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) catalyze functionally identical reactions in these two pathways, resulting in the formation of mono- or dimethoxylated lignin precursors. The activities of the two enzymes increase from the first to the sixth internode in stems of alfalfa (Medicago sativa L.), preceding the deposition of lignin. Alfalfa CCOMT is highly similar at the amino acid sequence level to the CCOMT from parsley, although it contains a six-amino acid insertion near the N terminus. Transcripts encoding both COMT and CCOMT are primarily localized to vascular tissue in alfalfa stems. Alfalfa CCOMT expressed in Escherichia coli catalyzes O-methylation of caffeoyl and 5-hydroxyferuloyl CoA, with preference for caffeoyl CoA. It has low activity against the free acids. COMT expressed in E. coli is active against both caffeic and 5-hydroxyferulic acids, with preference for the latter compound. Surprisingly, very little extractable O-methyltransferase activity versus 5-hydroxyferuloyl CoA is present in alfalfa stem internodes, in which relative O-methyltransferase activity against 5-hy-droxyferulic acid increases with increasing maturity, correlating with increased lignin methoxyl content.

Sorbitol-6-Phosphate Dehydrogenase Expression in Transgenic Tobacco1 : High Amounts of Sorbitol Lead to Necrotic Lesions

We analyzed transgenic tobacco (Nicotiana tabacum L.) expressing Stpd1, a cDNA encoding sorbitol-6-phosphate dehydrogenase from apple, under the control of a cauliflower mosaic virus 35S promoter. In 125 independent transformants variable amounts of sorbitol ranging from 0.2 to 130 μmol g−1 fresh weight were found. Plants that accumulated up to 2 to 3 μmol g−1 fresh weight sorbitol were phenotypically normal, with successively slower growth as sorbitol amounts increased. Plants accumulating sorbitol at 3 to 5 μmol g−1 fresh weight occasionally showed regions in which chlorophyll was partially lost, but at higher sorbitol amounts young leaves of all plants lost chlorophyll in irregular spots that developed into necrotic lesions. When sorbitol exceeded 15 to 20 μmol g−1 fresh weight, plants were infertile, and at even higher sorbitol concentrations the primary regenerants were incapable of forming roots in culture or soil. In mature plants sorbitol amounts varied with age, leaf position, and growth conditions. The appearance of lesions was correlated with high sorbitol, glucose, fructose, and starch, and low myo-inositol. Supplementing myo-inositol in seedlings and young plants prevented lesion formation. Hyperaccumulation of sorbitol, which interferes with inositol biosynthesis, seems to lead to osmotic imbalance, possibly acting as a signal affecting carbohydrate allocation and transport.

Expression of error-prone polymerases in BL2 cells activated for Ig somatic hypermutation

High affinity antibodies are generated in mice and humans by means of somatic hypermutation (SHM) of variable (V) regions of Ig genes. Mutations with rates of 10−5–10−3 per base pair per generation, about 106-fold above normal, are targeted primarily at V-region hot spots by unknown mechanisms. We have measured mRNA expression of DNA polymerases ι, η, and ζ by using cultured Burkitt's lymphoma (BL)2 cells. These cells exhibit 5–10-fold increases in heavy-chain V-region mutations targeted only predominantly to RGYW (R = A or G, Y = C or T, W = T or A) hot spots if costimulated with T cells and IgM crosslinking, the presumed in vivo requirements for SHM. An ∼4-fold increase pol ι mRNA occurs within 12 h when cocultured with T cells and surface IgM crosslinking. Induction of pols η and ζ occur with T cells, IgM crosslinking, or both stimuli. The fidelity of pol ι was measured at RGYW hot- and non-hot-spot sequences situated at nicks, gaps, and double-strand breaks. Pol ι formed T⋅G mispairs at a frequency of 10−2, consistent with SHM-generated C to T transitions, with a 3-fold increased error rate in hot- vs. non-hot-spot sequences for the single-nucleotide overhang. The T cell and IgM crosslinking-dependent induction of pol ι at 12 h may indicate an SHM “triggering” event has occurred. However, pols ι, η, and ζ are present under all conditions, suggesting that their presence is not sufficient to generate mutations because both T cell and IgM stimuli are required for SHM induction.

Aromatase (Cyp19) expression is up-regulated by targeted disruption of Dax1

DAX-1 [dosage-sensitive sex reversal, adrenal hypoplasia congenita (AHC) critical region on the X chromosome, gene 1] is an orphan nuclear receptor that represses transcription by steroidogenic factor-1 (SF-1), a factor that regulates expression of multiple steroidogenic enzymes and other genes involved in reproduction. Mutations in the human DAX1 gene (also known as AHC) cause the X-linked syndrome AHC, a disorder that is associated with hypogonadotropic hypogonadism also. Characterization of Dax1-deficient male mice revealed primary testicular defects that included Leydig cell hyperplasia (LCH) and progressive degeneration of the germinal epithelium, leading to infertility. In this study, we investigated the effect of Dax1 disruption on the expression profile of various steroidogenic enzyme genes in Leydig cells isolated from Dax1-deficient male mice. Expression of the aromatase (Cyp19) gene, which encodes the enzyme that converts testosterone to estradiol, was increased significantly in the Leydig cells isolated from mutant mice, whereas the expression of other proteins (e.g., StAR and Cyp11a) was not altered. In in vitro transfection studies, DAX-1 repressed the SF-1-mediated transactivation of the Cyp19 promoter but did not inhibit the StAR or Cyp11a promoters. Elevated Cyp19 expression was accompanied by increased intratesticular levels of estradiol. Administration of tamoxifen, a selective estrogen-receptor modulator, restored fertility to the Dax1-deficient male mice and partially corrected LCH, suggesting that estrogen excess contributes to LCH and infertility. Based on these in vivo and in vitro analyses, aromatase seems to be a physiologic target of Dax-1 in Leydig cells, and increased Cyp19 expression may account, in part, for the infertility and LCH in Dax1-deficient mice.

Expression and functional significance of VE-cadherin in aggressive human melanoma cells: Role in vasculogenic mimicry

We recently have introduced the term vasculogenic mimicry to describe the unique ability of aggressive melanoma tumor cells to form tubular structures and patterned networks in three-dimensional culture, which “mimics” embryonic vasculogenic networks formed by differentiating endothelial cells. In the current study, we address the biological significance of several endothelial-associated molecules (revealed by microarray analysis) with respect to expression and function in highly aggressive and poorly aggressive human cutaneous melanoma cell lines (established from the same patient). In a comparative analysis, CD31 was not expressed by any of the melanoma cell lines, whereas TIE-1 (tyrosine kinase with Ig and epidermal growth factor homology domains-1) was strongly expressed in the highly aggressive tumor cells with a low level of expression in one of the poorly aggressive cell lines. Vascular endothelial (VE)-cadherin was exclusively expressed by highly aggressive melanoma cells and was undetectable in the poorly aggressive tumor cells, suggesting the possibility of a vasculogenic switch. Down-regulation of VE-cadherin expression in the aggressive melanoma cells abrogated their ability to form vasculogenic networks and directly tested the hypothesis that VE-cadherin is critical in melanoma vasculogenic mimicry. These results highlight the plasticity of aggressive melanoma cells and call into question their possible genetic reversion to an embryonic phenotype. This finding could pose a significant clinical challenge in targeting tumor cells that may masquerade as circulating endothelial cells or other embryonic-like stem cells.

Tbx19, a tissue-selective regulator of POMC gene expression

Pituitary cell types arise in a temporally and spatially specific fashion, in response to combinatorial actions of transcription factors induced by transient signaling gradients. The critical transcriptional determinants of the two pituitary cell types that express the pro-opiomelanocortin (POMC) gene, the anterior lobe corticotropes, producing adrenocorticotropin, and the intermediate lobe melanotropes, producing melanocyte-stimulating hormone (MSHα), have remained unknown. Here, we report that a member of the T-box gene family, Tbx19, which is expressed only in the rostral ventral diencephalon and pituitary gland, commencing on e11.5, marks pituitary cells that will subsequently express the POMC gene and is capable of altering progression of ventral cell types and inducing adrenocorticotropin in rostral tip cells. It is suggested that Tbx19, depending on the presence of synergizing transcription factors, can activate POMC gene expression and repress the α glycoprotein subunit and thyroid-stimulating hormone β promoters.

Targeted expression of heme oxygenase-1 prevents the pulmonary inflammatory and vascular responses to hypoxia

Chronic hypoxia causes pulmonary hypertension with smooth muscle cell proliferation and matrix deposition in the wall of the pulmonary arterioles. We demonstrate here that hypoxia also induces a pronounced inflammation in the lung before the structural changes of the vessel wall. The proinflammatory action of hypoxia is mediated by the induction of distinct cytokines and chemokines and is independent of tumor necrosis factor-α signaling. We have previously proposed a crucial role for heme oxygenase-1 (HO-1) in protecting cardiomyocytes from hypoxic stress, and potent anti-inflammatory properties of HO-1 have been reported in models of tissue injury. We thus established transgenic mice that constitutively express HO-1 in the lung and exposed them to chronic hypoxia. HO-1 transgenic mice were protected from the development of both pulmonary inflammation as well as hypertension and vessel wall hypertrophy induced by hypoxia. Significantly, the hypoxic induction of proinflammatory cytokines and chemokines was suppressed in HO-1 transgenic mice. Our findings suggest an important protective function of enzymatic products of HO-1 activity as inhibitors of hypoxia-induced vasoconstrictive and proinflammatory pathways.

Two glucose transporters in Saccharomyces cerevisiae are glucose sensors that generate a signal for induction of gene expression.

Glucose is the preferred carbon source for most eukaryotic cells and has profound effects on many cellular functions. How cells sense glucose and transduce a signal into the cell is a fundamental, unanswered question. Here we describe evidence that two unusual glucose transporters in the yeast Saccharomyces cerevisiae serve as glucose sensors that generate an intracellular glucose signal. The Snf3p high-affinity glucose transporter appears to function as a low glucose sensor, since it is required for induction of expression of several hexose transporter (HXT) genes, encoding glucose transporters, by low levels of glucose. We have identified another apparent glucose transporter, Rgt2p, that is strikingly similar to Snf3p and is required for maximal induction of gene expression in response to high levels of glucose. This suggests that Rgt2p is a high glucose-sensing counterpart to Snf3p. We identified a dominant mutation in RGT2 that causes constitutive expression of several HXT genes, even in the absence of the inducer glucose. This same mutation introduced into SNF3 also causes glucose-independent expression of HXT genes. Thus, the Rgt2p and Snf3p glucose transporters appear to act as glucose receptors that generate an intracellular glucose signal, suggesting that glucose signaling in yeast is a receptor-mediated process.

A stable human-derived packaging cell line for production of high titer retrovirus/vesicular stomatitis virus G pseudotypes.

We have generated a human 293-derived retroviral packaging cell line (293GPG) capable of producing high titers of recombinant Moloney murine leukemia virus particles that have incorporated the vesicular stomatitis virus G (VSV-G) protein. To achieve expression of the retroviral gag-pol polyprotein, the precise coding sequences for gag-pol were introduced into a vector which utilizes totally nonretroviral signals for gene expression. Because constitutive expression of the VSV-G protein is toxic in 293 cells, we used the tetR/VP 16 transactivator and teto minimal promoter system for inducible, tetracycline-regulatable expression of VSV-G. After stable transfection of the 293GPG packaging cell line with the MFG.SnlsLacZ retroviral vector construct, it was possible to readily isolate stable virus-producing cell lines with titers approaching 10(7) colony-forming units/ml. Transient transfection of 293GPG cells using a modified version of MFG.SnlsLacZ, in which the cytomegalovirus IE promoter was used to drive transcription of the proviral genome, led to titers of approximately 10(6) colony-forming units/ml. The retroviral/VSV-G pseudotypes generated using 293GPG cells were significantly more resistant to human complement than commonly used amphotropic vectors and could be highly concentrated (> 1000-fold). This new packaging cell line may prove to be particularly useful for assessing the potential use of retroviral vectors for direct in vivo gene transfer. The design of the cell line also provides at least theoretical advantages over existing cell lines with regard to the possible release of replication-competent virus.

A novel cationic lipid greatly enhances plasmid DNA delivery and expression in mouse lung.

Effective gene therapy for lung tissue requires the use of efficient vehicles to deliver the gene of interest into lung cells. When plasmid DNA encoding chloramphenicol acetyltransferase (CAT) was administered intranasally to BALB/c mice without carrier lipids, CAT activity was detected in mouse lung extracts. Plasmid DNA delivered with optimally formulated commercially available transfection reagents expressed up to 10-fold more CAT activity in lung than observed with naked DNA alone. Liposome formulations consisting of (+/-)-N-(3-aminopropyl)-N,N-dimethyl-2,3-bis (dodecyloxy)-1-propanaminium bromide (GAP-DLRIE) plus the neutral colipid dioleoylphosphatidylethanolamine (DOPE) enhanced CAT expression by more than 100-fold relative to plasmid DNA alone. A single administration of GAP-DLRIE liposome-CAT DNA complexes to mouse lung elicited peak expression at days 1-4 posttransfection, followed by a gradual return to baseline by day 21 postadministration. Readministration of GAP-DLRIE liposome CAT complexes at day 21 led to another transient peak of reporter gene expression. Histological examination of lungs treated with GAP-DLRIE complexed beta-galactosidase DNA revealed that alveolar epithelial cells were the primary locus of expression and that up to 1% of all alveoli contained epithelial cells expressing the transgene.