Expression of a mouse metallothionein gene in transgenic plant tissues

Heterologous Expression of Human Metallothionein Gene HsMT1L Can Enhance the Tolerance of Tobacco (Nicotiana nudicaulis Watson) to Zinc and Cadmium

Metallothionein (MT) is a multifunctional inducible protein in animals, plants, and microorganisms. MT is rich in cysteine residues (10-30%), can combine with metal ions, has a low molecular weight, and plays an essential biological role in various stages of the growth and development of organisms. Due to its strong ability to bind metal ions and scavenge free radicals, metallothionein has been used in medicine, health care, and other areas. Zinc is essential for plant growth, but excessive zinc (Zn) is bound to poison plants, and cadmium (Cd) is a significant environmental pollutant. A high concentration of cadmium can significantly affect the growth and development of plants and even lead to plant death. In this study, the human metallothionein gene HsMT1L under the control of the CaMV 35S constitutive promoter was transformed into tobacco, and the tolerance and accumulation capacity of transgenic tobacco plants to Zn and Cd were explored. The results showed that the high-level expression of HsMT1L in tobacco could significantly enhance the accumulation of Zn²⁺ and Cd²⁺ in both the aboveground parts and the roots compared to wild-type tobacco plants and conferred a greater tolerance to Zn and Cd in transgenic tobacco. Subcellular localization showed that HsMT1L was localized to the nucleus and cytoplasm in the tobacco. Our study suggests that HsMT1L can be used for the phytoremediation of soil for heavy metal removal.

Differential pre-mRNA Splicing Alters the Transcript Diversity of Helitrons Between the Maize Inbred Lines

The propensity to capture and mobilize gene fragments by the highly abundant Helitron family of transposable elements likely impacts the evolution of genes in Zea mays. These elements provide a substrate for natural selection by giving birth to chimeric transcripts by intertwining exons of disparate genes. They also capture flanking exons by read-through transcription. Here, we describe the expression of selected Helitrons in different maize inbred lines. We recently reported that these Helitrons produce multiple isoforms of transcripts in inbred B73 via alternative splicing. Despite sharing high degrees of sequence similarity, the splicing profile of Helitrons differed among various maize inbred lines. The comparison of Helitron sequences identified unique polymorphisms in inbred B73, which potentially give rise to the alternatively spliced sites utilized by transcript isoforms. Some alterations in splicing, however, do not have obvious explanations. These observations not only add another level to the creation of transcript diversity by Helitrons among inbred lines but also provide novel insights into the cis-acting elements governing splice-site selection during pre-mRNA processing.

Discovery and expression analysis of alternative splicing events conserved among plant SR proteins

The high frequency of alternative splicing among the serine/arginine-rich (SR) family of proteins in plants has been linked to important roles in gene regulation during development and in response to environmental stress. In this article, we have searched and manually annotated all the SR proteins in the genomes of maize and sorghum. The experimental validation of gene structure by reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed, with few exceptions, that SR genes produced multiple isoforms of transcripts by alternative splicing. Despite sharing high structural similarity and conserved positions of the introns, the profile of alternative splicing diverged significantly between maize and sorghum for the vast majority of SR genes. These include many transcript isoforms discovered by RT-PCR and not represented in extant expressed sequence tag (EST) collection. However, we report the occurrence of various maize and sorghum SR mRNA isoforms that display evolutionary conservation of splicing events with their homologous SR genes in Arabidopsis and moss. Our data also indicate an important role of both 5' and 3' untranslated regions in the regulation of SR gene expression. These observations have potentially important implications for the processes of evolution and adaptation of plants to land.

Two novel arginine/serine (SR) proteins in maize are differentially spliced and utilize non-canonical splice sites

The serine-arginine (SR)-rich splicing proteins are highly conserved RNA binding nuclear phosphor-proteins that play important roles in both regular and alternative splicing. Here we describe two novel putative SR genes from maize, designated zmRSp31A and zmRSp31B. Both genes contain characteristic RNA binding motifs RNP-1 and RNP-2, a serine/arginine-rich (RS) domain and share significant sequence similarity to the Arabidopsis atRSp31 family of SR proteins. Both zmRSp31A and zmRSp31B produce multiple transcripts by alternative splicing, of which majority of the alternatively spliced transcripts utilize non-canonical splice sites. zmRSp31A and zmRSp31B produce at least six and four transcripts, respectively, of which only one corresponds to the wild type proteins for each gene. All the alternatively spliced transcripts of both the genes, with one exception, are predicted to encode small truncated proteins containing only the RNP-2 domain of their first RNA recognition motif and completely lack the carboxyl terminal RS domain. We provide evidence that some of the alternatively spliced transcripts of both genes are associated with polysomes and interact with the translational machinery.

An SC35-like protein and a novel serine/arginine-rich protein interact with Arabidopsis U1-70K protein

The U1 small nuclear ribonucleoprotein 70-kDa protein, a U1 small nuclear ribonucleoprotein-specific protein, has been shown to have multiple roles in nuclear precursor mRNA processing in animals. By using the C-terminal arginine-rich region of Arabidopsis U1-70K protein in the yeast two-hybrid system, we have identified an SC35-like (SR33) and a novel plant serine/arginine-rich (SR) protein (SR45) that interact with the plant U1-70K. The SR33 and SR45 proteins share several features with SR proteins including modular domains typical of splicing factors in the SR family of proteins. However, both plant SR proteins are rich in proline, and SR45, unlike most animal SR proteins, has two distinct arginine/serine-rich domains separated by an RNA recognition motif. By using coprecipitation assays we confirmed the interaction of plant U1-70K with SR33 and SR45 proteins. Furthermore, in vivo and in vitro protein-protein interaction experiments have shown that SR33 protein interacts with itself and with SR45 protein but not with two other members (SRZ21 and SRZ22) of the SR family that are known to interact with the Arabidopsis full-length U-70K only. A Clk/Sty protein kinase (AFC-2) from Arabidopsis phosphorylated four SR proteins (SR33, SR45, SRZ21, and SRZ22). Coprecipitation studies have confirmed the interaction of SR proteins with AFC2 kinase, and the interaction between AFC2 and SR33 is modulated by the phosphorylation status of these proteins. These and our previous results suggest that the plant U1-70K interacts with at least four distinct members of the SR family including SR45 with its two arginine/serine-rich domains, and the interaction between the SR proteins and AFC2 is modulated by phosphorylation. The interaction of plant U1-70K with a novel set of proteins suggests the early stages of spliceosome assembly, and intron recognition in plants is likely to be different from animals.

Purification and characterization of recombinant Caenorhabditis elegans metallothionein

The roundworm Caenorhabditis elegans adapted for survival at high concentrations of Cd(II) expresses two isoforms of metallothionein, CeMT-I and CeMT-II. To characterize one of these proteins CeMT-II was prepared as its Cd containing form by expressing its cDNA heterologously in Escherichia coli. The purified 63-amino-acid protein was identified as the desired product by ion-spray mass spectrometry and was found to resemble in most of its chemical and spectroscopic features the metallothioneins of other animal phyla. The recombinant protein contains a total of 18 cysteine residues and, as documented by electrophoresis and mass spectrometry, binds firmly six Cd ions through the cysteine's side chains. The (113)Cd NMR spectrum features six (113)Cd resonances. Their chemical shift positions between 615 and 675 ppm denote the existence of clusters of tetrahedrally coordinated cadmium thiolate complexes. The metal thiolate coordination dominates also the electronic far-UV absorption spectrum. It is characterized by a massive absorption profile with Cd thiolate shoulders at 255 and 235 nm. Upon replacement of Cd by Zn the profile was blue-shifted by 30 nm.

A splice site mutant of maize activates cryptic splice sites, elicits intron inclusion and exon exclusion, and permits branch point elucidation

DNA sequence analysis of the bt2-7503 mutant allele of the maize brittle-2 gene revealed a point mutation in the 5' terminal sequence of intron 3 changing GT to AT. This lesion completely abolishes use of this splice site, activates two cryptic splice sites, and alters the splicing pattern from extant splice sites. One activated donor site, located nine nt 5' to the normal splice donor site, begins with the dinucleotide GC. While non-consensus, this sequence still permits both trans-esterification reactions of pre-mRNA splicing. A second cryptic site located 23 nt 5' to the normal splice site and beginning with GA, undergoes the first trans-esterification reaction leading to lariat formation, but lacks the ability to participate in the second reaction. Accumulation of this splicing intermediate and use of an innovative reverse transcriptase-polymerase chain reaction technique (J. Vogel, R.H. Wolfgang, T. Borner [1997] Nucleic Acids Res 25: 2030-2031) led to the identification of 3' intron sequences needed for lariat formation. In most splicing reactions, neither cryptic site is recognized. Most mature transcripts include intron 3, while the second most frequent class lacks exon 3. Traditionally, the former class of transcripts is taken as evidence for the intron definition of splicing, while the latter class has given credence to the exon definition of splicing.

The AG dinucleotide terminating introns is important but not always required for pre-mRNA splicing in the maize endosperm

Previous RNA analysis of lesions within the 15 intron-containing Sh2 (shrunken2) gene of maize (Zea mays) revealed that the majority of these mutants affect RNA splicing. Here we decipher further two of these mutants, sh2-i (shrunken2 intermediate phenotype) and sh2-7460. Each harbors a G-to-A transition in the terminal nucleotide of an intron, hence destroying the invariant AG found at the terminus of virtually all nuclear introns. Consequences of the mutations, however, differ dramatically. In sh2-i the mutant site is recognized as an authentic splice site in approximately 10% of the primary transcripts processed in the maize endosperm. The other transcripts exhibited exon skipping and lacked exon 3. A G-to-A transition in the terminus of an intron was also found in the mutant sh2-7460, in this case intron 12. The lesion activates a cryptic acceptor site downstream 22 bp within exon 13. In addition, approximately 50% of sh2-7460 transcripts contain intron 2 and 3 sequences.

Effect of cadmium treatment on the expression of chimeric genes in transgenic tobacco seedlings and calli

Transgenic tobacco plants and calli bearing the bacterialuid A gene under the transcriptional control ofrbcS, mas and CaMV35S promoter(s) were exposed to different concentrations of cadmium. The transcriptional activity of the promoters was monitored using p-nitrophenyl β-D-g1ucuronide as a substrate for the β-glucuronidase (uidA) reporter enzyme. Therbc S promoter was repressed by high concentrations of cadmium. An induction of themas promoter was seen after cadmium treatment of seedlings but not calli. The activity of the CaMV35S promoter was unaffected by cadmium in both seedlings and calli.

Splicing of precursors to mRNA in higher plants: mechanism, regulation and sub-nuclear organisation of the spliceosomal machinery

The removal of introns from pre-mRNA transcripts and the concomitant ligation of exons is known as pre-mRNA splicing. It is a fundamental aspect of constitutive eukaryotic gene expression and an important level at which gene expression is regulated. The process is governed by multiple cis-acting elements of limited sequence content and particular spatial constraints, and is executed by a dynamic ribonucleoprotein complex termed the spliceosome. The mechanism and regulation of pre-mRNA splicing, and the sub-nuclear organisation of the spliceosomal machinery in higher plants is reviewed here. Heterologous introns are often not processed in higher plants indicating that, although highly conserved, the process of pre-mRNA splicing in plants exhibits significant differences that distinguish it from splicing in yeast and mammals. A fundamental distinguishing feature is the presence of and requirement for AU or U-rich intron sequence in higher-plant pre-mRNA splicing. In this review we document the properties of higher-plant introns and trans-acting spliceosomal components and discuss the means by which these elements combine to determine the accuracy and efficiency of pre-mRNA processing. We also detail examples of how introns can effect regulated gene expression by affecting the nature and abundance of mRNA in plants and list the effects of environmental stresses on splicing. Spliceosomal components exhibit a distinct pattern of organisation in higher-plant nuclei. Effective probes that reveal this pattern have only recently become available, but the domains in which spliceosomal components concentrate were identified in plant nuclei as enigmatic structures some sixty years ago. The organisation of spliceosomal components in plant nuclei is reviewed and these recent observations are unified with previous cytochemical and ultrastructural studies of plant ribonuleoprotein domains.

Cloning and characterization of two cDNA clones encoding seed-specific antimicrobial peptides from Mirabilis jalapa L

We have isolated and characterized two cDNA clones (designated MJ1 and MJ2) encoding the two Mirabilis jalapa antimicrobial peptides (Mj-AMP1 and Mj-AMP2, respectively), which were previously purified from seeds of this plant species (Cammue et al. (1992), J Biol Chem 267: 2228-2233). In both cases, the deduced amino acid sequences reveal the presence of a putative signal sequence preceding the mature peptide, indicating that the Mj-AMPs are expressed as preproteins. The Mj-AMP1- and Mj-AMP2-encoding genes are interrupted in their coding sequences by a single intron (380 bp and 900 bp for Mj-AMP1 and Mj-AMP2 genes, respectively). Southern blot analysis indicates that the Mj-AMP-encoding genes belong to a gene family of low complexity. Northern blot analysis suggests seed-specific expression of Mj-AMPs since transcripts of the expected size could only be detected in near-mature and in mature seeds of M. jalapa.

In vivo analysis of intron processing using splicing-dependent reporter gene assays

The mechanisms of intron recognition and processing have been well-studied in mammals and yeast, but in plants the biochemistry of splicing is not known and the rules for intron recognition are not clearly defined. To increase understanding of intron processing in plants, we have constructed new pairs of vectors, pSuccess and pFail, to assess the efficiency of splicing in maize cells. In the pFail series we use translation of pre-mRNA to monitor the amount of unspliced RNA. We inserted an ATG codon in the Bz2 (Bronze-2) intron in frame with luciferase: this construct will express luciferase activity only when splicing fails. In the pSuccess series the spliced message is monitored by inserting an ATG upstream of the Bz2 intron in frame with luciferase: this construct will express luciferase activity only when splicing succeeds. We show here, using both the wild-type Bz2 intron and the same intron with splice site mutations, that the efficiency of splicing can be estimated by the ratio between the luciferase activities of the vector pairs. We also show that mutations in the unique U-rich motif inside the intron can modulate splicing. In addition, a GC-rich insertion in the first exon increases the efficiency of splicing, suggesting that exons also play an important role in intron recognition and/or processing.

Purification and characterisation of recombinant sea urchin metallothionein expressed in Escherichia coli

Metallothioneins (MT) are metalloproteins expressed tissue specifically during the development of the sea urchin, Strongylocentrotus pururatus. To explore their structural and functional features and to compare them with those of the evolutionary distant mammalian MTs, one isoform (MTA) was obtained as the cadmium-containing form, from synthetic cDNA heterologously expressed in Escherichia coli. The purified protein was identified as the desired product by a combination of peptide-map analysis, amino acid sequence analysis and ion-spray mass spectroscopy. The existence of seven 113Cd NMR resonances revealed that the recombinant protein binds seven Cd ions/molecule. The position of the NMR resonances (605-695 ppm) and the electronic absorption features suggest that the sea urchin MTA, like the mammalian MTs, possesses tetrahedrally coordinated cadmium-thiolate clusters. With its large Stokes' radius, sea urchin MTA resembles the mammalian forms, suggesting a comparable elongated molecular shape. Measurements by spectrophotometric pH titration of cadmium binding by the recombinant protein suggest that it possesses two metal-thiolate clusters of distinctly different stability. At pH 7 the average apparent association constant for Cd2+ in the clusters is about 20-times weaker in sea urchin MTA than in rabbit MT-2.

3' splice site selection in dicot plant nuclei is position dependent

In contrast to mammalian and yeast systems, the mechanism for intron recognition and splice site selection in plant pre-mRNAs is poorly understood. Splice site sequences and putative branchpoint sequences are loosely conserved in plant introns compared with other eukaryotes. Perhaps to compensate for these variations, plant introns are significantly richer in adenosine and uridine residues than are their adjacent exons. To define elements critical for 3' splice site selection in dicotyledonous plant nuclei, pre-mRNA transcripts containing intron 3 of the maize Adh1 gene were expressed in Nicotiana benthamiana nuclei by using an autonomously replicating plant expression vector. Using a series of intron rearrangements which reposition the 3' intron-exon border, we demonstrate that the normal 3' splice site is defined in a position-dependent manner and that cryptic 3' splice sites within the intron are masked by the presence of a functional downstream 3' splice site. Disruption of the AU-rich elements upstream from the normal 3' splice site indicates that multiple AU elements between -66 and -6 cooperatively define the 3' boundary of the intron. These results are consistent with a model for plant intron recognition in which AU-rich elements spread throughout the length of the intron roughly define the intron boundaries by generating strong AU transition points. Functional 3' splice sites located downstream from these AU-rich sequences are preferentially selected over sites embedded within them.

3' splice site selection in dicot plant nuclei is position dependent

In contrast to mammalian and yeast systems, the mechanism for intron recognition and splice site selection in plant pre-mRNAs is poorly understood. Splice site sequences and putative branchpoint sequences are loosely conserved in plant introns compared with other eukaryotes. Perhaps to compensate for these variations, plant introns are significantly richer in adenosine and uridine residues than are their adjacent exons. To define elements critical for 3' splice site selection in dicotyledonous plant nuclei, pre-mRNA transcripts containing intron 3 of the maize Adh1 gene were expressed in Nicotiana benthamiana nuclei by using an autonomously replicating plant expression vector. Using a series of intron rearrangements which reposition the 3' intron-exon border, we demonstrate that the normal 3' splice site is defined in a position-dependent manner and that cryptic 3' splice sites within the intron are masked by the presence of a functional downstream 3' splice site. Disruption of the AU-rich elements upstream from the normal 3' splice site indicates that multiple AU elements between -66 and -6 cooperatively define the 3' boundary of the intron. These results are consistent with a model for plant intron recognition in which AU-rich elements spread throughout the length of the intron roughly define the intron boundaries by generating strong AU transition points. Functional 3' splice sites located downstream from these AU-rich sequences are preferentially selected over sites embedded within them.

Expression of maize Adh1 intron mutants in tobacco nuclei

In vivo and in vitro gene transfer experiments have suggested that the elements mediating intron recognition differ in mammalian, yeast and plant nuclei. Differences in the sequence dependencies, which also exist between dicotyledonous and monocotyledonous nuclei, have prevented some monocot introns from being spliced in dicot nuclei. To locate elements which modulate efficient recognition of introns in dicot nuclei, the maize Adh1 gene has been expressed in full-length and single intron constructs in Nicotiana benthamiana nuclei using an autonomously replicating plant expression vector. Quantitative PCR-Southern analyses indicate that the inefficient splicing of the maize Adh1 intron 1 (57% AU) in these dicot nuclei can be dramatically enhanced by increasing the degree of U1 snRNA complementarity at the 5' splice site. This indicates that the 5' splice site plays a significant role in defining the splicing efficiency of an intron in dicot nuclei and that, most importantly, the remainder of this monocot intron contains no elements which inhibit its accurate recognition in dicot nuclei. Deletions in intron 3 (66% AU) which effectively move the 3' boundary between AU-rich intron and GC-rich exon sequences strongly activate a cryptic upstream splice site; those which do not reposition this boundary activate a downstream cryptic splice site. This suggests that 3' splice site selection in dicot nuclei is extremely flexible and not dependent on strict sequence requirements but rather on the transition points between introns and exons. Our results are consistent with a model in which potential splice sites are selected if they are located upstream (5' splice site) or downstream (3' splice site) of AU transition points and not if they are embedded within AU-rich sequences.

Tissue partitioning of cadmium in transgenic tobacco seedlings and field grown plants expressing the mouse metallothionein I gene

Since agricultural crops contribute > 70% of human cadmium (Cd) intake, modification of crops to reduce accumulation of this pollutant metal during plant growth is desirable. Here we describe Cd accumulation characteristics of seedlings and field grown tobacco plants expressing the Cd-chelating protein, mouse metallothionein I. The objective of the transformation is to entrap Cd in roots as Cd-metallothionein and thereby reduce its accumulation in the shoot. Transformed and control seedlings were exposed for 15 days in liquid culture at a field soil-solution-like Cd concentration of 0.02 microM. Transformed seedlings of Nicotiana tabacum cultivar KY 14 contained about 24% lower Cd concentration in shoots and about 5% higher Cd concentration in roots than control seedlings. Dry weights of transformed and control tissues did not differ significantly. In the field in 1990, mature transformed N. tabacum cv. KY 14 plants exposed only to endogenous soil Cd contained about 14% lower leaf lamina Cd concentration than did controls. Differences were significant at the p < or = 0.1 level in 13 of 16 leaf positions. Leaf dry weight did not differ significantly but transformed field plants had 12% fewer leaves and were 9% shorter than the controls. Copper (Cu) concentration was significantly higher (ca10%) in the bottom nine leaf positions of transformed plants suggesting that reduced leaf number and plant height may be due to Cu deficiency or toxicity. Alternatively, somaclonal variation or gene position effects may be involved. No differences were found in zinc levels. With N. tabacum cv. Petit Havana, transformed seedlings contained no less Cd in shoots but 48% higher Cd concentration in roots.(ABSTRACT TRUNCATED AT 250 WORDS)

Insertion of non-intron sequence into maize introns interferes with splicing

Transposable element (TE) insertion into or near plant introns can cause intron skipping and alternative splicing events, resulting in reduced expression. To explore the impact of inserted sequences on splicing, we added non-intron sequence to two maize introns and tested these chimeric introns in a maize transient expression assay. Non-intron sequence inserted into Adh1-S intron 1 and actin intron 3 decreased expression from the luciferase reporter gene; the insertion sites tested were not in intron regions thought to be essential for splicing. Alternatively spliced mRNAs were not observed in transcripts derived from the insertion variants. In contrast, addition of an internal segment of an intron to Adh1-S intron 1 resulted in normal splice site selection and efficient processing. Because the normal intron sequence (including the conserved splice junctions) was retained in all constructs, we hypothesize that added non-intron sequence can interfere with intron recognition and/or splicing.