Engineered GFP as a vital reporter in plants

Use of green fluorescent protein to detect expression of an endopolygalacturonase gene of Colletotrichum lindemuthianum during bean infection

The 5' noncoding region of clpg2, an endopolygalacturonase gene of the bean pathogen Colletotrichum lindemuthianum, was fused to the coding sequence of a gene encoding a green fluorescent protein (GFP), and the construct was introduced into the fungal genome. Detection of GFP accumulation by fluorescence microscopy examination revealed that clpg2 was expressed at the early stages of germination of the conidia and during appressorium formation both in vitro and on the host plant.

Identification of a calmodulin-regulated Ca2+-ATPase in the endoplasmic reticulum

A unique subfamily of calmodulin-dependent Ca2+-ATPases was recently identified in plants. In contrast to the most closely related pumps in animals, plasma membrane-type Ca2+-ATPases, members of this new subfamily are distinguished by a calmodulin-regulated autoinhibitor located at the N-terminal instead of a C-terminal end. In addition, at least some isoforms appear to reside in non-plasma membrane locations. To begin delineating their functions, we investigated the subcellular localization of isoform ACA2p (Arabidopsis Ca2+-ATPase, isoform 2 protein) in Arabidopsis. Here we provide evidence that ACA2p resides in the endoplasmic reticulum (ER). In buoyant density sucrose gradients performed with and without Mg2+, ACA2p cofractionated with an ER membrane marker and a typical "ER-type" Ca2+-ATPase, ACA3p/ECA1p. To visualize its subcellular localization, ACA2p was tagged with a green fluorescence protein at its C terminus (ACA2-GFPp) and expressed in transgenic Arabidopsis. We collected fluorescence images from live root cells using confocal and computational optical-sectioning microscopy. ACA2-GFPp appeared as a fluorescent reticulum, consistent with an ER location. In addition, we observed strong fluorescence around the nuclei of mature epidermal cells, which is consistent with the hypothesis that ACA2p may also function in the nuclear envelope. An ER location makes ACA2p distinct from all other calmodulin-regulated pumps identified in plants or animals.

The green fluorescent protein targets secretory proteins to the yeast vacuole

The green fluorescent protein (GFP) was used as a marker to study the intracellular transport of vacuolar and secretory proteins in yeast. Therefore, the following gene constructs were expressed in Saccharomyces cerevisiae under control of the GAL1 promoter: GFP N-terminally fused to the yeast secretory invertase (INV-GFP), the plant vacuolar chitinase (CHN-GFP) and its secretory derivative (CHNDeltaVTP-GFP), which did not contain the vacuolar targeting peptide (VTP), both chitinase forms (CHN and CHNDeltaVTP), GFP without any targeting information and two secretory GFP variants with and without the VTP of chitinase (N-GFP-V and N-GFP). Whereas chitinase without VTP is accumulated in the culture medium the other gene products are retained inside the cell up to 48 h of induction. Independently of a known VTP they are transported to the vacuole, so far as they contain a signal peptide for entering the endoplasmic reticulum. This was demonstrated by confocal laser scanning microscopy, immunocytochemical analysis and subcellular fractionation experiments as well. The transport of the GFP fusion proteins is temporary delayed by a transient accumulation in electron-dense structures very likely derived from the ER, because they also contain the ER chaperone Kar2p/Bip. Our results demonstrate that GFP directs secretory proteins without VTP to the yeast vacuole, possibly by the recognition of an unknown vacuolar signal and demonstrates, therefore, a first limitation for the application of GFP as a marker for the secretory pathway in yeast.

A single amino acid, outside the AlcR zinc binuclear cluster, is involved in DNA binding and in transcriptional regulation of the alc genes in Aspergillus nidulans

In Aspergillus nidulans, the transcriptional activator AlcR mediates specific induction of a number of alc genes. The AlcR DNA-binding domain is a zinc binuclear cluster that differs from the other members of the Zn2Cys6 family in several respects. Of these, the most remarkable is its ability to bind in vitro as a monomer to single sites, whereas only repeated sites (direct or inverted) are necessary and functional in vivo. Deletion of the first five amino acids (following the N-terminal methionine) upstream of the AlcR zinc cluster or mutation of a single residue, Arg-6, impairs the AlcR in vitro binding mainly to symmetrical sites. In vivo, the same mutations result in the inability of A. nidulans to grow on ethanol. The alc- phenotype results from a drastic decrease in activation of its own transcription and, in addition, that of the two structural genes, alcA and aldA, required for ethanol oxidation. This defect seems to be correlated to the inability of the Arg-6 AlcR mutant protein to bind to AlcR palindrome targets, which are essential in the three alc promoters. AlcR shows a unique pattern of binding and of transactivation among the Zn2Cys6 family.

Light regulates alternative splicing of hydroxypyruvate reductase in pumpkin

Hydroxypyruvate reductase (HPR) is a leaf peroxisomal enzyme that functions in the glycolate pathway of photorespiration in plants. We have obtained two highly similar cDNAs for pumpkin HPR (HPR1 and HPR2). It has been revealed that two HPR mRNAs might be produced by alternative splicing from a single type of pre-mRNA. The HPR1 protein, but not the HPR2 protein, was found to have a targeting sequence into leaf peroxisomes at the C-terminus, suggesting that alternative splicing controls the subcellular localization of the two HPR proteins. Immunoblot analysis and subcellular fractionation experiments showed that HPR1 and HPR2 proteins are localized in leaf peroxisomes and the cytosol, respectively. Moreover, indirect fluorescence microscopy and analyses of transgenic tobacco cultured cells and Arabidopsis thaliana expressing fusion proteins with green fluorescent protein (GFP) revealed the different subcellular localizations of the two HPR proteins. Both mRNAs were induced developmentally and by light, but with quantitative differences. Almost equal amounts of the mRNAs were detected in pumpkin cotyledons grown in darkness, but treatment with light greatly enhanced the production of HPR2 mRNA. These findings indicate that light regulates alternative splicing of HPR mRNA, suggesting the presence of a novel mechanism of mRNA maturation, namely light-regulated alternative splicing, in higher plants.

GFP variants for multispectral imaging of living cells

Unlike enzyme markers, green fluorescent protein can be visualized at high resolution in living cells using confocal microscopy. The images are not prone to fixation or staining artifacts, and can be of exceptional clarity. Moreover, the activities of living cells, such as cytoplasmic streaming, are clearly evident during microscopy. Ordinarily, movement within a sample is a nuisance, placing constraints on the use of sometimes lengthy techniques for noise reduction during confocal microscopy, such as frame averaging. However, it is possible to monitor dynamic events by time-lapse confocal microscopy, and this combination of a vital fluorescent reporter with high-resolution optical techniques shows much promise for use in cell biological and physiological experiments. Genetic systems such as that of Arabidopsis provide a large resource of potentially informative mutants, and there has been much recent improvement in techniques for determining the molecular basis of a particular phenotype. The use of fluorescent proteins will provide further tools for examining the biology of mutant cells. The precision with which particular cellular structures can be decorated with GFP and the ease with which subcellular traffic can be monitored indicate that this approach will be very useful for cell biological and physiological observations, particularly for detailed examination of plant mutant phenotypes.

Regulation of serine biosynthesis in Arabidopsis. Crucial role of plastidic 3-phosphoglycerate dehydrogenase in non-photosynthetic tissues

In plants, Ser is synthesized through a couple of pathways. 3-Phosphoglycerate dehydrogenase (PGDH), the first enzyme that is involved in the phosphorylated pathway of Ser biosynthesis, is responsible for the oxidation of 3-phosphoglycerate to phosphohydroxypyruvate. Here we report the first molecular cloning and characterization of PGDH from Arabidopsis thaliana. Sequence analysis of cDNA and a genomic clone revealed that the PGDH gene is composed of three exons, encoding a 623-amino acid polypeptide (66, 453 Da). The deduced protein, containing three of the most conserved regions in the NAD-dependent 2-hydroxyacid dehydrogenase family, has 38-39% identity to its animal and bacterial counterparts. The presence of an N-terminal signal sequence for translocation into plastids was confirmed by particle-gun bombardment experiments using green fluorescence protein as a reporter protein for subcellular localization. Southern hybridization analysis and restriction fragment length polymorphism mapping indicated that PGDH is a single-copy gene that is mapped to the upper arm of chromosome 1. Northern hybridization analysis indicated preferential expression of PGDH mRNA in root tissues of light-grown plants, suggesting that the phosphorylated pathway of Ser biosynthesis plays an important role in supplying Ser to non-photosynthetic tissues. The recombinant enzyme overproduced in Escherichia coli displayed hyperbolic kinetics with respect to 3-phosphoglycerate and NAD+.

Isoform-dependent differences in feedback regulation and subcellular localization of serine acetyltransferase involved in cysteine biosynthesis from Arabidopsis thaliana

Serine acetyltransferase (SATase; EC 2.3.1.30), which catalyzes the formation of O-acetyl-L-serine (OAS) from acetyl-CoA and L-serine, plays a regulatory role in the biosynthesis of cysteine by its property of feedback inhibition by cysteine in bacteria and certain plants. Three cDNA clones encoding SATase isoforms (SAT-c, SAT-p, and SAT-m) have been isolated from Arabidopsis thaliana. However, the significance of the feedback regulation has not yet been clear in these different isoforms of SATase from A. thaliana. We constructed the overexpression vectors for cDNAs encoding three SATase isoforms of A. thaliana and analyzed the inhibition of SATase activity by cysteine using the recombinant SATase proteins. In the case of SAT-c, the activity was feedback-inhibited by a low concentration of cysteine (the concentration that inhibits 50% activity; IC50 = 1.8 microM). By contrast, SAT-p and SAT-m were feedback inhibition-insensitive isozymes. We also determined the subcellular localization of three SATase isozymes by the transient expression of fusion proteins of each SATase N-terminal region with jellyfish green fluorescent protein (GFP) in 4-week-old Arabidopsis leaves. The SAT-c-GFP fusion protein was stayed in cytosol, whereas SAT-p-GFP and SAT-m-GFP fusion proteins were localized in chloroplasts and in mitochondria, respectively. These results suggest that these three SATase isoforms, which are localized in the different organelles, are subjected to different feedback regulation, presumably so as to play the particular roles for the production of OAS and cysteine in Arabidopsis cells. Regulatory circuit of cysteine biosynthesis in the plant cells is discussed.

Molecular characterization of plastidic phosphoserine aminotransferase in serine biosynthesis from Arabidopsis

Serine biosynthesis in plants proceeds by two pathways; a photorespiratory pathway which is associated with photorespiration and a pathway from phosphoglycerate. A cDNA encoding plastidic phosphoserine aminotransferase (PSAT) which catalyzes the formation of phosphoserine from phosphohydroxypyruvate has been isolated from Arabidopsis thaliana. Genomic DNA blot analysis indicated that this enzyme is most probably encoded by a single gene and is mapped on the lower arm of chromosome 4. The deduced protein contains an N-terminal extension exhibiting the general features of a plastidic transit peptide, which was confirmed by subcellular organelle localization using GFP (green flourescence protein). Northern analysis indicated preferential expression of PSAT in roots of light-grown plants, supporting the idea that the phosphorylated pathway may play an important role in supplying the serine requirement of plants in non-green tissues. In situ hybridization analysis of PSAT revealed that the gene is generally expressed in all types of cells with a significantly higher amount in the meristem tissue of root tips.

Suppression of auxin signal transduction by a MAPK cascade in higher plants

The plant hormone auxin activates many early response genes that are thought to be responsible for diverse aspects of plant growth and development. It has been proposed that auxin signal transduction is mediated by a conserved signalling cascade consisting of three protein kinases: the mitogen-activated protein kinase (MAPK), MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK). Here we show that a specific plant MAPKKK, NPK1, activates a MAPK cascade that leads to the suppression of early auxin response gene transcription. A mutation in the kinase domain abolishes NPK1 activity, and the presence of the carboxy-terminal domain diminishes the kinase activity. Moreover, the effects of NPK1 on the activation of a MAPK and the repression of early auxin response gene transcription are specifically eliminated by a MAPK phosphatase. Transgenic tobacco plants overexpressing the NPK1 kinase domain produced seeds defective in embryo and endosperm development. These results suggest that auxin sensitivity may be balanced by antagonistic signalling pathways that use a distinct MAPK cascade in higher plants.

Visualization of Pit-1 transcription factor interactions in the living cell nucleus by fluorescence resonance energy transfer microscopy

The pituitary-specific transcription factor Pit-1 forms dimers when interacting with specific DNA elements and has been shown to associate with several other nuclear proteins. Recently, techniques have become available that allow visualization of protein-protein interactions as they occur in single living cells. In this study, the technique of fluorescence resonance energy transfer (FRET) microscopy was used to visualize the physical interactions of Pit-1 proteins fused to spectral variants of the jellyfish green fluorescent protein (GFP) that emit green or blue light [blue fluorescent protein (BFP)]. An optimized imaging system was used to discriminate fluorescence signals from single cells coexpressing the BFP- and GFP-fusion proteins, and the contribution of spectral overlap to background fluorescence detected in the FRET images was established. Energy transfer signals from living cells expressing a fusion protein in which GFP was tethered to BFP by short protein linker was used to demonstrate acquisition of FRET signals. Genetic vectors encoding GFP- and BFP-Pit-1 proteins were prepared, and biological function of the fusion proteins was confirmed. FRET microscopy of HeLa cells coexpressing the GFP- and BFP-Pit-1 demonstrated energy transfer, which required the two fluorophores to be separated by less than 100 A. Biochemical studies previously demonstrated that Pit-1 physically interacts with both c-Ets-1 and the estrogen receptor. FRET imaging of cells coexpressing BFP-Pit-1 and GFP-Ets-1 demonstrated energy transfer between these fusion proteins, a result consistent with their association in the nucleus of these living cells. In contrast, there was no evidence for energy transfer between the BFP-Pit-1 and an estrogen receptor-GFP fusion proteins. It is likely that the FRET imaging approach described here can be applied to many different protein-partner pairs in a variety of cellular contexts.

RNA-mediated trans-activation of transcription from a viral RNA

The red clover necrotic mosaic virus genome is composed of two single-stranded RNA components, RNA-1 and RNA-2. The viral capsid protein is translated from a subgenomic RNA (sgRNA) that is transcribed from genomic RNA-1. Here, a 34-nucleotide sequence in RNA-2 is shown to be required for transcription of sgRNA. Mutations that prevent base-pairing between the RNA-1 subgenomic promoter and the 34-nucleotide trans-activator prevent expression of a reporter gene. A model is proposed in which direct binding of RNA-2 to RNA-1 trans-activates sgRNA synthesis. This RNA-mediated regulation of transcription is unusual among RNA viruses, which typically rely on protein regulators.

Green fluorescent protein as a reporter of transcription and protein localization in fungi

Green fluorescent protein (GFP) is a versatile and powerful tool for analysis of diverse biological processes. The recent development of GFP variants with altered spectral properties and altered codon composition has allowed efficient expression of GFP in a number of fungal species. GFP has been successfully used to analyze transcription regulation as well as protein and organelle localization, and promises to give an unprecedented view into the dynamic subcellular processes that shape the fungal cell.

Complementation of the mpg1 mutant phenotype in Magnaporthe grisea reveals functional relationships between fungal hydrophobins

The functional relationship between fungal hydrophobins was studied by complementation analysis of an mpg1(-) gene disruption mutant in Magnaporthe grisea. MPG1 encodes a hydrophobin required for full pathogenicity of the fungus, efficient elaboration of its infection structures and conidial rodlet protein production. Seven heterologous hydrophobin genes were selected which play distinct roles in conidiogenesis, fruit body development, aerial hyphae formation and infection structure elaboration in diverse fungal species. Each hydrophobin was introduced into an mpg1(-) mutant by transformation. Only one hydrophobin gene, SC1 from Schizophyllum commune, was able partially to complement mpg1(-) mutant phenotypes when regulated by its own promoter. In contrast, six of the transformants expressing hydrophobin genes controlled by the MPG1 promoter (SC1 and SC4 from S.commune, rodA and dewA from Aspergillus nidulans, EAS from Neurospora crassa and ssgA from Metarhizium anisopliae) could partially complement each of the diverse functions of MPG1. Complementation was always associated with partial restoration of a rodlet protein layer, characteristic of the particular hydrophobin being expressed, and with hydrophobin surface assembly during infection structure formation. This provides the first genetic evidence that diverse hydrophobin-encoding genes encode functionally related proteins and suggests that, although very diverse in amino acid sequence, the hydrophobins constitute a closely related group of morphogenetic proteins.

GFP expression in Drosophila tissues: time requirements for formation of a fluorescent product

We analyzed the acquisition of fluorescence of a GFP fusion protein expressed from gene constructs designed to regulate either the time of translation of the protein or the time of gene transcription. Both gene constructs expressed a fusion protein with wild-type GFP appended to the N-terminus of the Bicoid (Bcd) transcription factor. One gene construct, P[gfp-bcd] expressed an mRNA that was initially translated at the time of egg deposition. GFP-Bcd fluorescence could first be detected in anterior nuclei in the embryo at syncytial blastoderm stage, within 1.5-2.5 h after translation commenced. The second gene construct, P[hs-gfp-bcd], utilized the hsp70 promoter to regulate the induction of gene expression. In larval salivary glands, GFP-Bcd fluorescence could first be detected in nuclei after a 40-min recovery period following a 10-min heat shock induction of gene expression. These estimates of the time required for the acquisition of a fluorescent GFP product in Drosophila tissues are less than suggested by previously published reports.

The ROOT HAIRLESS 1 gene encodes a nuclear protein required for root hair initiation in Arabidopsis

The epidermis of Arabidopsis wild-type primary roots, in which some cells grow hairs and others remain hairless in a position-dependent manner, has become an established model system to study cell differentiation. Here we present a molecular analysis of the RHL1 (ROOT HAIRLESS 1) gene that, if mutated, prevents the formation of hairs on primary roots and causes a seedling lethal phenotype. We have cloned the RHL1 gene by use of a T-DNA-tagged mutant and found that it encodes a protein that appears to be plant specific. The predicted RHL1 gene product is a small hydrophilic protein (38.9 kD) containing putative nuclear localization signals and shows no significant homology to any known amino acid sequence. We demonstrate that a 78-amino-acid sequence at its amino terminus is capable of directing an RHL1-GFP fusion protein to the nucleus. The RHL1 transcript is present throughout the wild-type plant and in suspension culture cells, but in very low amounts, suggesting a regulatory function for the RHL1 protein. Structural evidence suggests a role for the RHL1 gene product in the nucleolus. We have examined the genetic relationship between RHL1 and GL2, an inhibitor of root hair initiation in non-hair cells. Our molecular and genetic data with double mutants, together with the expression analysis of a GL2 promoter-GUS reporter gene construct, indicate that the RHL1 gene acts independently of GL2.

Mitochondrial localization of a NADP-dependent [corrected] isocitrate dehydrogenase isoenzyme by using the green fluorescent protein as a marker

In this work, we describe the isolation of a new cDNA encoding an NADP-dependent isocitrate dehydrogenase (ICDH). The nucleotide sequence in its 5' region gives a deduced amino acid sequence indicative of a targeting peptide. However, even if this cDNA clearly encodes a noncytosolic ICDH, it is not possible to say from the targeting peptide sequence to which subcellular compartment the protein is addressed. To respond to this question, we have transformed tobacco plants with a construct containing the entire targeting signal-encoding sequence in front of a modified green fluorescent protein (GFP) gene. This construct was placed under the control of the cauliflower mosaic virus 35S promoter, and transgenic tobacco plants were regenerated. At the same time, and as a control, we also have transformed tobacco plants with the same construct but lacking the nucleotide sequence corresponding to the ICDH-targeting peptide, in which the GFP is retained in the cytoplasm. By optical and confocal microscopy of leaf epiderm and Western blot analyses, we show that the putative-targeting sequence encoded by the cDNA addresses the GFP exclusively into the mitochondria of plant cells. Therefore, we conclude that this cDNA encodes a mitochondrial ICDH.

Isolation and characterization of tilapia (Oreochromis mossambicus) insulin-like growth factors gene and proximal promoter region

To understand the molecular mechanism which controls the transcription of the insulin-like growth factors (IGFs) gene, we have cloned and sequenced the cDNA for the proximal promoter region of the tilapia IGFs gene and have characterized its activity by chloramphenicol acetyltransferase (CAT) transient transfected expression assays. Tilapia (Oreochromis mossambicus) IGF-I cDNA (549 bp) was amplified by PCR from single-stranded cDNA of growth hormone (GH)-induced liver RNA using a pair of oligonucleotides specific for fish IGF-I as amplification primers. Tilapia IGF-I and IGF-II 5' termini were analyzed by rapid amplification of cDNA 5' ends (5'RACE). Analysis of the 5'RACE results revealed two transcription start sites in IGF-I and one transcription start site in IGF-II. Different fragments of the 5' flanking region were transfected into human lung adenocarcinoma cells. In the cell line, maximum promoter activity was located in the distal 657 basepairs of the IGF-I 5' flanking region and in the distal 450 basepairs of the IGF-II 5' flanking region. The in vivo actions of the IGFs promoter on developmental stage expression were investigated further in transgenic zebrafish in which an IGFs promoter-driven green fluorescent protein (GFP) encoding the cDNA transgene was microinjected into embryos. Morphologic and RT-PCR studies of the transgenic zebrafish indicated that IGF-I promoter-driven GFP transcripts appeared for the first time in the 1-K-cell stage and the IGF-II promoter-driven GFP transcripts appeared for the first time in the 32-cell stage. Fluorescent (GFP) distribution was apparent within 48 h in IGF-II-transgenic zebrafish embryos, especially in eye, muscle, corpuscle, floor plate, horizontal myoseptum, yolk sac extension, and yolk sac. These results indicate that the IGF-I and IGF-II promoters are active in tissue and in a development-specific manner. Our findings also indicate that the IGF-II promoter influences the growth of fish embryos earlier than does IGF-I, and IGF-II has higher levels of expression than does IGF-I. These results suggest that the IGF-II promoter plays a growth factor role in teleost embryo development.

Characterization of a calmodulin-binding transporter from the plasma membrane of barley aleurone

We have used Arabidopsis calmodulin (CaM) covalently coupled to horseradish peroxidase to screen a barley aleurone cDNA expression library for CaM binding proteins. The deduced amino acid sequence of one cDNA obtained by this screen was shown to be a unique protein of 702 amino acids with CaM and cyclic nucleotide binding domains at the carboxyl terminus and high similarity to olfactory and K+ channels. This cDNA was designated HvCBT1 (Hordeum vulgare CaM binding transporter). Hydropathy plots of HvCBT1 showed the presence of six putative transmembrane domains, but sequence alignment indicated a pore domain that was unlike the consensus domains in K+ and olfactory channels. Expression of a subclone of amino acids 482-702 in Escherichia coli generated a peptide that bound CaM. When a fusion protein of HvCBT1 and green fluorescent protein was expressed in barley aleurone protoplasts, fluorescence accumulated in the plasma membrane. Expression of HvCBT1 in the K+ transport deficient Saccharomyces cerevisiae mutant CY162 showed no rescue of the mutant phenotype. However, growth of CY162 expressing HvCBT1 with its pore mutated to GYGD, the consensus sequence of K+ channels, was compromised. We interpret these data as indicating that HvCBT1 acts to interfere with ion transport.

Plant-adapted green fluorescent protein is a versatile vital reporter for gene expression, protein localization and mitosis in the filamentous fungus, Aspergillus nidulans

Green fluorescent protein (GFP) is a useful reporter to follow the in vivo behaviour of proteins, but the wild-type gfp gene does not function in many organisms, including many plants and filamentous fungi. We show that codon-modified forms of gfp, produced for use in plants, function effectively in Aspergillus nidulans both as gene expression reporters and as vital reporters for protein location. To demonstrate the use of these modified gfps as reporter genes we have used fluorescence to follow ethanol-induced GFP expression from the alcA promoter. Translational fusions with the modified gfp were used to follow protein location in living cells; plant ER-retention signals targeted GFP to the endoplasmic reticulum, whereas fusion to the GAL4 DNA-binding domain targeted it to the nucleus. Nuclear-targeted GFP allowed real-time observation of nuclear movement and division. These modified gfp genes should provide useful markers to follow gene expression, organelle behaviour and protein trafficking in real time.

Leaf-specifically expressed genes for polypeptides destined for chloroplasts with domains of sigma70 factors of bacterial RNA polymerases in Arabidopsis thaliana

Genes for sigma-like factors of bacterial-type RNA polymerase have not been characterized from any multicellular eukaryotes, although they probably play a crucial role in the expression of plastid photosynthesis genes. We have cloned three distinct cDNAs, designated SIG1, SIG2, and SIG3, for polypeptides possessing amino acid sequences for domains conserved in sigma70 factors of bacterial RNA polymerases from the higher plant Arabidopsis thaliana. Each gene is present as one copy per haploid genome without any additional sequences hybridized in the genome. Transient expression assays using green fluorescent protein demonstrated that N-terminal regions of the SIG2 and SIG3 ORFs could function as transit peptides for import into chloroplasts. Transcripts for all three SIG genes were detected in leaves but not in roots, and were induced in leaves of dark-adapted plants in rapid response to light illumination. Together with results of our previous analysis of tissue-specific regulation of transcription of plastid photosynthesis genes, these results indicate that expressed levels of the genes may influence transcription by regulating RNA polymerase activity in a green tissue-specific manner.

High-Titer Retroviral Vectors Containing the Enhanced Green Fluorescent Protein Gene for Efficient Expression in Hematopoietic Cells

Retroviral vectors constitute the most efficient system to deliver and integrate foreign genes into mammalian cells. We have developed a producer cell line that yields high titers of amphotropic retroviral vectors carrying the enhanced green fluorescent protein (EGFP) gene, a codon humanized, red-shifted variant of the green fluorescent protein (GFP) gene, which can be used as a selectable marker. We have used a hybrid vector that has been shown to efficiently drive gene expression in hematopoietic cells. Virtually all murine and human cell lines and primary human hematopoietic cells tested were transduced with varying efficiency after incubation with vector-containing supernatants. Human CD34+ cells obtained from cord blood or aphereses products were transduced using a protocol that involves daily addition of vector-containing supernatants for 6 consecutive days. At day 6, up to 16% of the cells expressed EGFP, as assessed by flow cytometry. Sorted EGFP-expressing cells were able to produce fluorescent hematopoietic colonies. EGFP's main advantages are its fast flow cytometry determination and the possibility of cell sorting and simultaneous evaluation of the transduction efficiency along with other phenotypic markers.

Use of the green fluorescent protein and its mutants in quantitative fluorescence microscopy

We have investigated properties relevant to quantitative imaging in living cells of five green fluorescent protein (GFP) variants that have been used extensively or are potentially useful. We measured the extinction coefficients, quantum yields, pH effects, photobleaching effects, and temperature-dependent chromophore formation of wtGFP, alphaGFP (F99S/M153T/V163A), S65T, EGFP (F64L/S65T), and a blue-shifted variant, EBFP (F64L/S65T/Y66H/Y145F). Absorbance and fluorescence spectroscopy showed little difference between the extinction coefficients and quantum yields of wtGFP and alphaGFP. In contrast, S65T and EGFP extinction coefficients made them both approximately 6-fold brighter than wtGFP when excited at 488 nm, and EBFP absorbed more strongly than the wtGFP when excited in the near-UV wavelength region, although it had a much lower quantum efficiency. When excited at 488 nm, the GFPs were all more resistant to photobleaching than fluorescein. However, the wtGFP and alphaGFP photobleaching patterns showed initial increases in fluorescence emission caused by photoconversion of the protein chromophore. The wtGFP fluorescence decreased more quickly when excited at 395 nm than 488 nm, but it was still more photostable than the EBFP when excited at this wavelength. The wtGFP and alphaGFP were quite stable over a broad pH range, but fluorescence of the other variants decreased rapidly below pH 7. When expressed in bacteria, chromophore formation in wtGFP and S65T was found to be less efficient at 37 degrees C than at 28 degrees C, but the other three variants showed little differences between 37 degrees C and 28 degrees C. In conclusion, no single GFP variant is ideal for every application, but each one offers advantages and disadvantages for quantitative imaging in living cells.

Applications of the green fluorescent protein in cell biology and biotechnology

The recent emergence of an autofluorescent protein, the green fluorescent protein (GFP), has opened the door for the convenient use of intact living cells and organisms as experimental systems in fields ranging from cell biology to biomedicine. We present an overview of some of the major applications of GFP, namely its use in protein tagging and in monitoring gene expression as well as its potential in a variety of biological screens.

Reporter gene expression for monitoring gene transfer

The use of reporters such as green fluorescent protein (GFP) and firefly luciferase permit highly sensitive and nondestructive monitoring of gene transfer and expression. Modifications in GFP which increase intensity and thermostability, as well as alter its spectral qualities, have facilitated the use of GFP in a variety of gene transfer methods. Improvements in imaging technologies and their increased application in biological research have allowed the expanded use of luciferase-based reporters in gene transformation, particularly in genetic screens and in monitoring temporal changes in gene expression.

Identification of a motor protein required for filamentous growth in Ustilago maydis

The phytopathogenic fungus Ustilago maydis exists in two stages, the yeast-like haploid form and the filamentous dikaryon. Both pathogenicity and dimorphism are genetically controlled by two mating-type loci, with only the filamentous stage being pathogenic on corn. We have identified two genes (kin1 and kin2) encoding motor proteins of the kinesin family. Kin1 is most similar to the human CENP-E gene product, while Kin2 is most closely related to the conventional kinesin Nkin of Neurospora crassa. Deletion mutants of kin1 had no discernible phenotype; delta kin2 mutants, however, were severely affected in hyphal extension and pathogenicity. The wild-type dikaryon showed rapid tip growth, with all the cytoplasm being moved to the tip compartment. Left behind are septate cell wall tubes devoid of cytoplasm. In delta kin2 mutants, dikaryotic cells were formed after cell fusion, but these hyphal structures remained short and filled with cytoplasm. A functional green fluorescent protein (GFP)-Kin2 fusion was generated and used to determine the localization of the motor protein by fluorescence microscopy. Inspection of the hyphal tips by electron microscopy revealed a characteristic accumulation of darkly stained vesicles which was absent in mutant cells. We suggest that the motor protein Kin2 is involved in organizing this specialized growth zone at the hyphal tip, probably by affecting the vectorial transport of vesicles.

FLUORESCENCE MICROSCOPY OF LIVING PLANT CELLS

Since its inception, light microscopy has shown the elegance and subtlety with which function is expressed in the form of the cells, tissues, and organs of the plant. Recently, light microscopy has seen a resurgence in use fueled by advances in microscope design and computer-based image analysis. The structural resolution afforded by static, fixed samples is being increasingly supplemented by approaches using fluorescent analogs and selective fluorescent indicators, which visualize the dynamic processes in living, functioning cells. This review describes some of these approaches and discusses how they are taking us a step closer to viewing the intricate complexity with which plants organize and regulate their functions down to the subcellular level.

Removal of a cryptic intron and subcellular localization of green fluorescent protein are required to mark transgenic Arabidopsis plants brightly

The green fluorescent protein (GFP) from the jellyfish Aequorea victoria is finding wide use as a genetic marker that can be directly visualized in the living cells of many heterologous organisms. We have sought to express GFP in the model plant Arabidopsis thaliana, but have found that proper expression of GFP is curtailed due to aberrant mRNA processing. An 84-nt cryptic intron is efficiently recognized and excised from transcripts of the GFP coding sequence. The cryptic intron contains sequences similar to those required for recognition of normal plant introns. We have modified the codon usage of the gfp gene to mutate the intron and to restore proper expression in Arabidopsis. GFP is mainly localized within the nucleoplasm and cytoplasm of transformed Arabidopsis cells and can give rise to high levels of fluorescence, but it proved difficult to efficiently regenerate transgenic plants from such highly fluorescent cells. However, when GFP is targeted to the endoplasmic reticulum, transformed cells regenerate routinely to give highly fluorescent plants. These modified forms of the gfp gene are useful for directly monitoring gene expression and protein localization and dynamics at high resolution, and as a simply scored genetic marker in living plants.

Optimized codon usage and chromophore mutations provide enhanced sensitivity with the green fluorescent protein

The green fluorescent protein (GFP) from Aequorea victoria is a versatile reporter protein for monitoring gene expression and protein localization in a variety of cells and organisms. Despite many early successes using this reporter, wild type GFP is suboptimal for most applications due to low fluorescence intensity when excited by blue light (488 nm), a significant lag in the development of fluorescence after protein synthesis, complex photoisomerization of the GFP chromophore and poor expression in many higher eukaryotes. To improve upon these qualities, we have combined a mutant of GFP with a significantly larger extinction coefficient for excitation at 488 nm with a re-engineered GFP gene sequence containing codons preferentially found in highly expressed human proteins. The combination of improved fluorescence intensity and higher expression levels yield an enhanced GFP which provides greater sensitivity in most systems.