A stable bifunctional antisense transcript inhibiting gene expression in transgenic plants

Antisense Transcription in Plants: A Systematic Review and an Update on cis-NATs of Sugarcane

Initially, natural antisense transcripts (NATs, natRNAs, or asRNAs) were considered repressors; however, their functions in gene regulation are diverse. Positive, negative, or neutral correlations to the cognate gene expression have been noted. Although the first studies were published about 50 years ago, there is still much to be investigated regarding antisense transcripts in plants. A systematic review of scientific publications available in the Web of Science databases was conducted to contextualize how the studying of antisense transcripts has been addressed. Studies were classified considering three categories: “Natural antisense” (208), artificial antisense used in “Genetic Engineering” (797), or “Natural antisense and Genetic Engineering”-related publications (96). A similar string was used for a systematic search in the NCBI Gene database. Of the 1132 antisense sequences found for plants, only 0.8% were cited in PubMed and had antisense information confirmed. This value was the lowest when compared to fungi (2.9%), bacteria (2.3%), and mice (54.1%). Finally, we present an update for the cis-NATs identified in Saccharum spp. Of the 1413 antisense transcripts found in different experiments, 25 showed concordant expressions, 22 were discordant, 1264 did not correlate with the cognate genes, and 102 presented variable results depending on the experiment.

Down-regulation of specific members of the glutamine synthetase gene family in alfalfa by antisense RNA technology

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH3 with glutamate to produce glutamine. In plants GS is an octameric enzyme and is located either in the cytoplasm (GS1) or in the chloroplast (GS2). Two distinct classes of GS1 genes with unique 3'-untranslated region (3'UTR) have been identified in alfalfa. We have demonstrated that the two classes exhibit differential expression pattern in the different plant organs suggesting different functional roles for the different isozymes. To determine the functional significance of the two classes of GS1 genes in alfalfa, we have utilized antisense gene constructs aimed specifically at the 3'UTR of the two GS1 genes and introduced them individually into alfalfa. Our data show that the gene constructs are effective in lowering the corresponding transcript level very effectively though there were organ-specific differences in the level of reduction. No transcript corresponding to the antisense gene construct was detected in any of the alfalfa transformants though they accumulated to significant levels in transgenic tobacco containing the same construct. This suggests that the antisense transcript was not stable in the presence of the homologous target sequence. Transgenic alfalfa with up to 80% reduction in the transcript level corresponding to each gene class, however, showed no reduction in GS activity or GS1 polypeptide level. The results suggest that GS1 mRNA levels are not rate-limiting for GS1 polypeptide synthesis and that GS levels are controlled both at the transcriptional and translational/post-translational level.

Drought-induced responses in plant cells

Plants subjected to water stress undergo numerous physiological and metabolic changes. A general decrease in photosynthetic rate is among the most common responses. This is due to a programmed process involving the closure of stomata and reduction in the activity of photosynthetic enzymes. The plant hormone abscisic acid plays an important role in this process. Accumulation of compatible solutes, during water stress, is thought to be an adaptive response which has been developed by some plant species. Engineering the genes involved in the synthesis of these compounds, into nonaccumulating plants, has demonstrated promising results for genetic improvement of drought tolerance. Drought stress induces alteration of gene expression. A large number of genes which are upregulated by water stress have been isolated and characterized. Proteins encoded by some of these genes share several characteristics. The biochemical role of most of these gene products is unknown, but potential adaptive functions have been suggested. Abscisic acid is involved in the regulation of some of these genes.

Antisense suppression of S-RNase expression in Nicotiana using RNA polymerase II- and III-transcribed gene constructs

In the Solanaceae, self-incompatibility is controlled by a single, multi-allelic ('S') locus. One product of this locus is a ribonuclease, the S-RNase, which is expressed predominantly in mature pistils and has recently been shown to cause allele-specific pollen rejection in transgenic plants. Hybrid Nicotiana plumbaginifolia x N. alata plants were used to test the effects of antisense suppression of the SA2-RNase from N. alata using three different gene constructs: two driven by RNA polymerase II-transcribed promoters, and the third, containing a truncated soybean tRNA (met-i) gene, transcribed by RNA polymerase III. All three constructs caused suppression of S-RNase activity in the transgenic plants. Unexpectedly, the CaMV 35S promoter was more effective for antisense suppression than the tissue specific tomato ChiP promoter. Antisense suppression of S-RNase correlated with low sense SA2 transcript levels and high antisense SA2 transcript levels. Untransformed hybrids that contained the N. alata SA2 allele were incompatible with N. alata SA2 pollen, while transgenic plants with suppressed SA2 gene expression accepted the pollen. The utility of this hybrid plant system for studying some aspects of antisense gene suppression is discussed.

Transformation of the wild tomatoLycopersicon chilense Dun. byAgrobacterium tumefaciens

Leaf disc transformation-regeneration technique was applied to the drought tolerant wild relative of cultivated tomato,Lycopersicon chilense, using a plasmid construct which contained the coding sequences of neomycin phosphotransferase (NPTII) and chloramphenicol acetyltransferase (CAT) genes. The two genotypes used, LA2747 and LA1930, showed a distinct difference in their aptitude to transformation; a higher success rate was obtained for the first genotype in every stage of the process. Shoots were formed on the regeneration medium containing 100 μg/ml kanamycin through direct or indirect organogenesis. Root formation became only possible when the concentration of kanamycin was reduced to 50 μg/ml. Expression of chloramphenicol acetyltransferase gene was observed in all of the kanamycin-screened plants after they matured; the activity of the gene was absent or low in some of the young plants. The presence of the CAT gene in transgenic plants was further confirmed by Southern blot analysis. Although transgenic plants grew to maturity, they did not produce fruit, owing to the self incompatibility ofL. chilense.

Parameters affecting the activity of antisense RNA sequences in tobacco protoplasts

Plasmids containing various fragments of the β-glucuronidase (GUS) gene were placed in antisense orientation downstream of the cauliflower mosaic virus 35S promoter and cotransfected with a 35S-gus construct into tobacco mesophyll protoplasts. None of the partial-length sequences were as effective as the full-length sequence in reducing GUS activity. The presence of a polyadenylation sequence downstream of the antisense sequence had an enhancing effect. The activity of the antisense sequence was largely affected by the incubation temperature of the transfected protoplasts. The chloramphenicol acetyltransferase (CAT) gene was fused to the gus coding sequence. When this construct was cotransfected with an antisense sequence directed against CAT, GUS activity was reduced. The implications of these results for the design and uses of antisense sequences are discussed.

A plant scaffold attached region detected close to a T-DNA integration site is active in mammalian cells

Integration of foreign genes into plant genomes by the Agrobacterium T-DNA transfer system has been considered to occur at random. It has been speculated that the chromosomal structure of the integration site might affect the expression pattern of the introduced genes. To gain insight into the molecular structure of T-DNA integration sites and its possible impact on gene expression, we have examined plant DNA sequences in the vicinity of T-DNA borders. Analysis of a transgenic petunia plant containing a chloramphenicol acetyltransferase (CAT) gene regulated by the hemoglobin promoter (PAR) from Parasponia andersonii revealed a scaffold attachment region (SAR) close to one T-DNA end. In addition to having strong binding affinities for both animal and plant nuclear scaffolds this petunia SAR element is as active in mammalian cells as the authentic elements from mammalian sources.

Tenascin-X: a novel extracellular matrix protein encoded by the human XB gene overlapping P450c21B

A human gene termed XB overlaps the P450c21B gene encoding steroid 21-hydroxylase and encodes a protein that closely resembles extracellular matrix proteins. Sequencing of phage and cosmid clones and of cDNA fragments shows that the XB gene spans 65 kb of DNA, consisting of 39 exons that encode a 12-kb mRNA. The predicted protein of over 400 kD consists of five distinct domains: a signal peptide, a hydrophobic domain containing three heptad repeats, a series of 18.5 EGF-like repeats, 29 fibronectin type III repeats, and a carboxy-terminal fibrinogen-like domain. Because the structure of the protein encoded by the XB gene closely resembles tenascin, we term this protein tenascin-X (TN-X), and propose a simplified nomenclature system for the family of tenascins. RNase protection experiments show that the TN-X transcript is expressed ubiquitously in human fetal tissues, with the greatest expression in the fetal testis and in fetal skeletal, cardiac, and smooth muscle. Two adrenal-specific transcripts, P450c21B (steroid 21-hydroxylase) and Y (an untranslated transcript) overlap the XB gene on the complementary strand of DNA, yielding a unique array of overlapping transcripts: a "polygene." In situ hybridization histochemistry experiments show that the TN-X transcript and the P450c21 and Y transcripts encoded on the complementary DNA strand are all expressed in the same cells of the human adrenal cortex. Genetic data suggest that TN-X may be essential for life.

Inhibition of tobacco NADH-hydroxypyruvate reductase by expression of a heterologous antisense RNA derived from a cucumber cDNA: implications for the mechanism of action of antisense RNAs

Tobacco plants were genetically transformed to generate antisense RNA from a gene construct comprised of a full-length cucumber NADH-dependent hydroxypyruvate reductase (HPR) cDNA placed in reverse orientation between the cauliflower mosaic virus 35S promoter and a nopaline synthase termination/polyadenylation signal sequence. In vivo accumulation of antisense HPR RNA within eight independent transgenic tobacco plants resulted in reductions of up to 50% in both native HPR activity and protein accumulation relative to untransformed tobacco plants (mean transgenote HPR activity = 67% wild type, mean transgenote HPR protein = 63% wild type). However, in contrast to previous reports describing antisense RNA effects in plants, production of the heterologous HPR antisense RNA did not systematically reduce levels of native tobacco HPR mRNA (mean transgenote HPR mRNA level = 135% wild type). Simple regression comparison of the steady-state levels of tobacco HPR mRNA to those of HPR antisense RNA showed a weak positive correlation (r value of 0.548, n = 9; n is wild type control plus eight independent transformants; significant at 85% confidence level), supporting the conclusion that native mRNA levels were not reduced within antisense plants. Although all transgenic antisense plants examined displayed an apparent reduction in both tobacco HPR protein and enzyme activity, there is no clear correlation between HPR activity and the amount of either sense (r = 0.267, n = 9) or antisense RNA (r = 0.175, n = 9). This compares to a weak positive correlation between HPR mRNA levels and the amount of HPR activity observed in wild-type SR1 tobacco plants (r = 0.603, n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

The Anti-nptII Gene (A Potential Negative Selectable Marker for Plants)

An efficient negative selection procedure is crucial to the isolation of rare homologous recombinants in gene targeting. Although gene targeting is a common practice in lower eukaryotes and is becoming routine in mammals, its application to plants has not been achieved. In this report, we have evaluated an antisense construct against the neomycin phosphotransferase gene (nptII) as a negative selectable marker. The anti-nptII gene construct was able to suppress nptII expression both transiently and in transformed tobacco (Nicotiana tabacum) calli. A construct was made which includes both a hygromycin-resistance gene and the sense plus antisense genes for neomycin phosphotransferase. Hygromy-cin-resistant calli were obtained after Agrobacterium-mediated transformation. Subsequently, hygromycin-resistant calli were tested for kanamycin sensitivity. The growth on kanamycin medium of calli harboring both the sense and antisense gene constructs was retarded, whereas that of control calli transformed with only the sense nptII gene was not inhibited. Southern blot analysis confirmed the presence of both nptII and anti-nptII genes. Northern blot analyses revealed that antisense transcripts of the nptII gene were made and that the level of sense transcripts was greatly reduced in transgenic calli. These results suggest that the anti-nptII gene could potentially be used as a negative selectable marker for gene targeting in plants.

Restoration of fertility by antisense RNA in genetically engineered male sterile tobacco plants

Transgenic tobacco plants (Nicotiana tabacum L.) expressing the rolC gene of Agrobacterium rhizogenes under the transcriptional control of the 35S RNA promoter are male sterile. When these plants are genetically crossed with others containing the rolC gene linked in antisense orientation to the 35S RNA promoter, hybrid progeny display restoration of male fertility. Moreover, hybrid progeny are revertant for other features of the rolC phenotype, such as restoration of plant height, leaf pigment content and female fertility. The level of restoration of the characteristics of untransformed tobacco appeared to be independent of the steady-state level of antisense RNA. Addition of six transcriptional enhancer sequences upstream of the 35S transcriptional start region in the antisense construct led to a higher steady-state level of antisense RNA than that produced using a promoter linked to a single enhancer sequence. However no significant difference was observed in the level of attenuation of the rolC phenotype in the progeny of crosses with either one or six transcriptional enhancers linked to the antisense rolC gene. Antisense constructs comprising only 189 bp of the rolC 5' coding region appeared less efficient in attenuating the rolC phenotype than those including the whole rolC coding region as well as its 3' untranslated region. Furthermore, results from experiments on light-controlled rolC gene expression indicate that microsporogenesis is sensitive to rolC gene action during the early stages of flower development.

Modulation of glutamine synthetase gene expression in tobacco by the introduction of an alfalfa glutamine synthetase gene in sense and antisense orientation: molecular and biochemical analysis

A glutamine synthetase (GS) cDNA isolated from an alfalfa cell culture cDNA library was found to represent a cytoplasmic GS. The full-length alfalfa GS1 coding sequence, in both sense and antisense orientation and under the transcriptional control of the cauliflower mosaic virus 35S promoter, was introduced into tobacco. Leaves of tobacco plants transformed with the sense construct contained greatly elevated levels of GS transcript and GS polypeptide which assembled into active enzyme. Leaves of the plants transformed with the antisense GS1 construct showed a significant decrease in the level of both GS1 and GS2 polypeptides and GS activity, but did not show any significant decrease in the level of endogenous GS mRNA. We have proposed that antisense inhibition using a heterologous antisense GS RNA occurs at the level of translation. Our results also suggest that the post-translational assembly of GS subunits into a holoenzyme requires an additional factor(s) and is under regulatory control.

Subcellular location of delta-pyrroline-5-carboxylate reductase in root/nodule and leaf of soybean

The expression of Delta(1)-pyrroline-5-carboxylate reductase (P5CR) gene was found to be higher in soybean root nodules than in leaves and roots, and its expression in roots appeared to be osmoregulated (AJ Delauney, DPS Verma [1990] Mol Gen Genet 221: 299-305). P5CR was purified to homogeneity as a monomeric protein of 29 kilodaltons by overexpression of a soybean P5CR cDNA clone in Escherichia coli. The pH optimum of the purified P5CR was altered by increasing the salt concentration, and maximum enzyme activity was attainable at a lower pH under high salt (0.2-1 molar NaCl). Kinetic studies of the purified enzyme suggested that nicotinamide adenine dinucleotide phosphate(+) inhibited P5CR activity, whereas nicotinamide adenine dinucleotide(+) did not. Subcellular fractionation and antibodies raised against purified soybean P5CR were used to investigate location of the enzyme in different parts of soybean as well as in leaves of transgenic tobacco plants synthesizing soybean P5CR. P5CR activity was present in cytoplasm of soybean roots and nodules as well as in leaves, but in leaves, about 15% of the activity was detected in the plastid fraction. The location of P5CR was further confirmed by western blot assay of the proteins from cytosol and plastid fractions of different parts of the plant. Expression of soybean nodule cytosolic P5CR in transgenic tobacco under the control of cauliflower mosaic virus 35S promoter led to the accumulation of this protein exclusively in the cytoplasm, suggesting that the chloroplastic activity may be due to the presence of a plastid form of the enzyme. The different locations of P5CR in root and leaf suggested that proline may be synthesized in different subcellular compartments in root and leaf. Proline concentration was not significantly increased in transgenic plants exhibiting high level P5CR activity, indicating that reduction of P5C is not a rate-limiting step in proline production.

Strategies for expression of foreign genes in plants. Potential use of engineered viruses

Advances in gene transfer techniques for higher plants have already permitted important achievements towards crop protection and improvement using recombinant DNA technology. Besides plant genetic engineering, the possible use of plant viruses to express foreign genes could be of considerable interest to plant biotechnology. However, insuring containment of engineered viruses for environmental use is an important safety issue that must be addressed.

Inhibition of the expression of the gene for granule-bound starch synthase in potato by antisense constructs

Granule-bound starch synthase [GBSS; EC 24.1.21] determines the presence of amylose in reserve starches. Potato plants were transformed to produce antisense RNA from a gene construct containing a full-length granule-bound starch synthase cDNA in reverse orientation, fused between the cauliflower mosaic virus 35S promoter and the nopaline synthase terminator. The construct was integrated into the potato genome by Agrobacterium rhizogenes-mediated transformation. Inhibition of GBSS activity in potato tuber starch was found to vary from 70% to 100%. In those cases where total suppression of GBSS activity was found both GBSS protein and amylose were absent, giving rise to tubers containing amylose-free starch. The variable response of the transformed plants indicates that position effects on the integrated sequences might be important. The results clearly demonstrate that in tubers of potato plants which constitutively synthesize antisense RNA the starch composition is altered.

Effect of heat on the nutritional quality and safety of soybean cultivars

To evaluate whether soybean strains with reduced levels of trypsin inhibitors have enhanced nutritional and safety characteristics, we measured protease inhibitor content of a standard cultivar (Williams 82) and an isoline (L81-4590) lacking the Kunitz trypsin inhibitor, using enzyme inhibition assays and enzyme-linked immunosorbent assays (ELISA). Less heat was needed to inactivate the remaining trypsin inhibitory activity of the isoline than that of the standard soybean cultivar. In fact, autoclaving (steam heating at 121 degrees C) of the isoline for 20 min resulted in a near zero level of trypsin inhibitor activity, while 20% remained in the Williams 82 sample. Feeding studies with rats showed that the raw soy flour prepared from the isoline was nutritionally superior to the raw flour prepared from the standard variety, as measured by PER and pancreatic weights. Since the content of amino and fatty acids of the flours from both strains was identical and the hemagglutinating activities were within a factor of 2, the increased PER was likely due to the lower level of trypsin inhibitory activity in the isoline. Steam heating the flours for up to 30 min at 121 degrees C progressively increased the PER for both strains. Preliminary screening of several accessions from the USDA Soybean Germplasm Collection showed considerable variation in the content of trypsin inhibitors, sulfur amino acids, and lectins. The BBI content of these cultivars, determined by chymotrypsin inhibition assays, was identical to that found by ELISA. The results indicate that further screening studies could lead to the discovery of soybeans which yield flour that is safe and nutritious, with minimal need for heating.

Heat-inducible hygromycin resistance in transgenic tobacco

We have constructed a chimaeric gene consisting of the promoter of the soybean heat shock (hs) gene Gmhsp17, 6-L, the coding region of a hygromycin phosphotransferase (hpt) gene, and the termination sequence of the nopaline synthase (nos) gene. This gene fusion was introduced into tobacco by Agrobacterium-mediated gene transfer. Heat-inducible synthesis of mRNA was shown by northern hybridization, and translation of this RNA into a functional protein was indicated by plant growth on hygromycin-containing media in a temperature-dependent fashion. One hour incubation at 40 degrees C per day, applied for several weeks, was sufficient to express the resistant phenotype in transgenic plants containing the chimaeric hs-hpt gene. These data suggest that the hygromycin resistance gene is functional and faithfully controlled by the soybean hs promoter. The suitability of these transgenic plants for selection of mutations that alter the hs response is discussed.

Regulation of plant gene expression by antisense RNA

Regulation of gene expression by antisense RNA was first discovered as a naturally-occurring phenomenon in bacteria. Recently natural antisense RNAs have been found in a variety of eukaryotic organisms; their in vivo function is, however, obscure. Deliberate expression of antisense RNA in animal and plant systems has lead to successful down-regulation of specific genes. We will review the current status of antisense gene action in plant systems. The recent discovery that 'sense' genes are able to mimic the action of antisense genes indicates that (anti)sense genes must operate by mechanisms other than RNA-RNA interaction.

Organ-specific modulation of gene expression in transgenic plants using antisense RNA

We have shown leaf-specific inhibition GUS gene expression in transgenic Nicotiana plants using an antisense RNA with a 41-base homology spanning the translation start codon of the gene. GUS was expressed from the nominally constitutive 35S promoter and the antisense RNA was expressed from the light-regulated ca/b promoter of Arabidopsis thaliana. A range of GUS inhibition from 0 to 100% was obtained by screening a small population of transgenic plants and the specific levels of inhibition observed were stably inherited in two generations. An antiGUS 'gene' dosage effect was observed in plants which were homozygous for antiGUS. RNA detection results suggest that duplex formation with the 41 base pair antiGUS RNA destabilized the GUS mRNA and that an excess of antisense RNA was not required. Our results demonstrate the potential of antisense RNA as a strategy for obtaining plant mutants, especially 'down mutations' in essential genes where only a short 5' sequence of the mRNA is required. They also suggest that the 'position effect' on gene expression could be used in conjunction with an antisense RNA strategy to provide a versatile approach for crop improvement.

A soybean gene encoding delta 1-pyrroline-5-carboxylate reductase was isolated by functional complementation in Escherichia coli and is found to be osmoregulated

We have isolated several cDNA clones encoding delta 1-pyrroline-5-carboxylate reductase (P5CR, L-proline: NAD(P)+ 5-oxidoreductase, EC 1.5.1.2) which catalyzes the terminal step in proline biosynthesis, by direct complementation of a proC mutation in Escherichia coli with an expression library of soybean root nodule cDNA. The library was constructed in the lambda ZapII vector, converted to a plasmid library by in vivo excision of recombinant pBluescript phagemids, and used for transformation of the E. coli mutant. Complementing plasmids contained inserts of about 1.2 kb which hybridized to a 1.3 kb RNA transcript in nodules, uninfected roots and leaves. DNA sequence analysis of one full length cDNA clone showed that it encodes a 28 586 Mr polypeptide with 39% amino acid identity to the E. coli P5CR sequence. Genomic analysis showed that there are two to three copies of the P5CR gene in the soybean genome. The steady-state level of P5CR mRNA in root nodules was twice as high as in uninfected roots and about five times higher than in leaves. Subjecting young seedlings to osmotic stress by watering with 400 mM NaCl resulted in an almost six-fold increase in the level of root P5CR mRNA, suggesting that this gene may be osmoregulated.

The conditional inhibition of gene expression in cultured Drosophila cells by antisense RNA

Genes producing antisense RNA are becoming important tools for the selective inhibition of gene expression. Experiments in different biological systems, targeting different mRNAs have yielded diverse results with respect to the success of the technique and its mechanism of action. We have examined the potential of three antisense genes, whose transcription is driven by a Drosophila metallothionein promoter, to inhibit the expression of alcohol dehydrogenase (ADH) or a microtubule associated protein (205K MAP) in cultured Drosophila cells. Expression of ADH was significantly reduced upon induction of the anti-ADH genes. The ADH mRNA does not appear to be destabilized by the presence of antisense RNA but rather exists at similar levels in hybrid form. Hybrids are detected with both spliced and unspliced ADH RNA. In contrast to these results, antisense genes producing antisense RNA in great excess to 205K MAP mRNA, which is itself far less abundant than the ADH mRNA, failed to show any inhibition of 205K MAP expression.

Antisense RNA inhibition of beta-glucuronidase gene expression in transgenic tobacco plants

Antisense RNA was used to specifically inhibit the expression of a GUS gene introduced in a transgenic plant. A tobacco transformant containing a single intact copy of the GUS gene and showing relatively high constitutive levels of GUS activity (GUS +) was re-transformed with an Agrobacterium Ti-derived binary vector containing an antisense version of this reporter gene. The sense and antisense GUS genes were each under the regulation of the CaMV 35S promoter. Re-transformed plants contained 1-5 copies of the antisense construct and all showed a greater than 90% reduction in GUS activity relative to the original GUS + plant. This reduction in GUS activity correlated closely with the levels of GUS enzyme and steady state GUS mRNA observed in these plants. The relatively low levels of sense and antisense GUS transcripts found in the re-transformed plants may indicate a rapid degradation of the RNA:RNA duplex in the cell.

Both RNA level and translation efficiency are reduced by anti-sense RNA in transgenic tobacco

The effect of anti-sense RNA on the expression of the bialaphos resistance (bar) gene which encodes phosphinothricin acetyl transferase (PAT), was analysed in tobacco. Transient expression studies revealed that an anti-bar RNA with sequence complementarity to the complete bar coding region, inhibits PAT synthesis. To quantify the phenomenon, SR1 tobacco cells were transformed twice to introduce first a hybrid bar gene with a reporter gene and in a second instance an anti-bar gene. A first cycle transformant and a double transformant derived herefrom in which PAT synthesis was reduced to only 8%, were studied in detail. The interference of the anti-sense gene with the expression of the bar gene is manifested at least two levels. First, the bar mRNA steady state level is significantly reduced relative to the parental whereas the transcript level of the reporter gene is unchanged. Comparison of bar mRNA levels in total and single stranded (ss) RNA preparations demonstrated that little if any stably base-pairing bar and anti-bar RNA accumulates. Secondly, a three fold reduction of PAT synthesis per bar mRNA is observed. This supposes that because of unstable interactions with the complementary anti-bar RNA either a substantial part of the bar mRNA detected does not enter the cytoplasm and/or that in the cytoplasm the bar mRNA is less efficiently translated. It is not clear if or how the reduced bar mRNA level is related to such unstable interactions.