Leader length and secondary structure modulate mRNA function under conditions of stress

Human and murine FMR-1: alternative splicing and translational initiation downstream of the CGG-repeat

Fragile X syndrome is associated with massive expansion of a CGG trinucleotide repeat within the FMR-1 gene and transcriptional silencing of the gene due to abnormal methylation. Partial cDNA sequence of the human FMR-1 has been reported. We report here the isolation and characterization of cDNA clones encoding the murine homologue, fmr-1, which exhibit marked sequence identity with the human gene, including the conservation of the CGG repeat. A conserved ATG downstream of the CGG repeat in human and mouse and an in-frame stop codon in other human 5' cDNA sequences demarcate the FMR-1 coding region and confine the CGG repeat to the 5' untranslated region. We also present evidence for alternative splicing of the FMR-1 gene in mouse and human brain and show that one of these splicing events alters the FMR-1 reading frame, predicting isoforms with novel carboxy termini.

Translation of the placenta growth factor mRNA is severely affected by a small open reading frame localized in the 5' untranslated region

The recently identified placenta growth factor gene (PIGF) code for a protein related to the vascular permeability factor (VPF). We present evidence indicating that expressing of this gene could be regulated at the post-transcriptional level. The region upstream to the coding region of PIGF mRNA contains a small open reading frame (ORF), potentially coding for a peptide of 15 amino acids. The translation of different constructs in reticulocyte and wheat germ lysates as well as in COS-1 and CV-1 cells indicates that this short region is a translational inhibitory element since mutations in its two potential initiator codons increase PIGF synthesis in vivo. Using RNAse protection assay, we demonstrate that the PIGF mRNAs obtained from human term placenta and JEG choriocarcinoma cell line have a complete 5' untranslated region and, consequently, also the above mentioned small ORF. Finally, the analysis of a bovine PIGF genomic clone reveals that this small ORF is strongly conserved with respect to both putative peptide sequences and distance from the PIGF coding region.

Sequence analysis of translationally controlled maternal mRNAs from Urechis caupo

Fertilization of Urechis caupo oocytes stimulates dramatic changes in the pattern of protein synthesis. This shift is brought about entirely through selective translation of the large pool of maternal mRNAs synthesized and stored during oogenesis. My laboratory has identified cDNA clones to more than 20 different Urechis maternal mRNAs. These have been used to determine whether the complementary mRNAs are translated in oocytes or embryos, and to analyze the polyadenylation status of the mRNAs at different stages. For 14 of the mRNAs, multiple, overlapping cDNA clones were isolated, and the complete sequence of the mRNA molecule was determined. Of these 14 mRNAs, half are from the subset that is translated in growing and full-grown oocytes, but not in embryos. These 7 mRNAs have poly(A) tails before fertilization. The other 7 are from the subset that is not translated at any time before fertilization, and has very short poly(A) tails in oocytes. After fertilization these mRNAs are recruited onto polysomes and extensively polyadenylated. The sequence data from the two classes of maternal mRNAs was compared in an attempt to identify consensus sequences that could regulate translation directly, or indirectly, by controlling polyadenylation or secondary structure formation. Two features of the sequences correlate very well with the translation and polyadenylation of the different mRNAs--the identity of the base immediately preceding the AUG start codon, and the presence of the sequences UUUUA and UUUUUA in the 3' untranslated region.

Translation inhibition by an mRNA coding region secondary structure is determined by its proximity to the AUG initiation codon

In the present study we investigate the impact of highly stable coding region secondary structures on mRNA translation efficiency. By introducing antisense segments into the 3'non-translated region of human alpha-globin mRNA we are able to synthesize a series of transcripts in which site-specific secondary structures are introduced without altering the primary structure of the 5' non-translated region, the coding region, or the encoded protein product. Coding region duplexes in close proximity to the AUG initiation codon are found to inhibit translation severely to a degree equal to that of a duplex that extends into the 5' non-translated region. In contrast, mRNAs containing duplexes positioned further 3' in the coding region translate at levels that are significantly higher although are still below those of native alpha-globin mRNA. The primary determinant of translation inhibition by coding region duplexes appears to be the proximity of the duplex to the AUG initiation codon and reflects a parallel inhibition of monosome formation. These data demonstrate that extensive coding region secondary structures suppress translation to a minimal or to a substantial degree depending on their distance from the initiation codon.

Isolation of cDNA clones encoding rat glial fibrillary acidic protein: expression in astrocytes and in Schwann cells

Glial fibrillary acidic protein (GFAP) expressed by astrocytes in the central nervous system (CNS) has been extensively characterized but the molecular identity of related molecules in the peripheral nervous system (PNS) remains unclear. To examine possible structural differences between CNS and PNS GFAP, we have isolated cDNA clones for rat GFAP from both cultured astrocyte and Schwann cell libraries. Nucleotide sequence analysis indicated that the PNS and CNS GFAP clones contained identical coding regions, with a predicted protein product of 430 amino acids. However, the 5'-untranslated region of clone rGFA15, isolated from the Schwann cell library, was longer than that predicted for brain-derived GFAP mRNA. Primer extension analysis of RNA isolated from the RT4-D6 Schwann cell line indicated that the start site for PNS GFAP mRNA lies 169 bases upstream from that used in the CNS. In addition, tryptic peptide mapping of GFAP prepared from cultured astrocytes and Schwann cells revealed one major peptide fragment present in CNS GFAP but absent from PNS GFAP. These results suggest structural differences between GFAP in these two cell types, at both the nucleic acid and protein level, and are consistent with previous observations of immunochemical differences existing between CNS and PNS GFAP.

Identification of proteins associating with poly(A)-binding-protein mRNA

Synthesis of poly(A)-binding protein is regulated at the translational level. We have investigated the binding of proteins to this mRNA on the premise that the protein(s) of the mRNP complex may be involved in regulating the expression of the mRNA. We found the first 243 nucleotides of the 5' untranslated region to contain sequences essential for RNP formation. A large, single-stranded bulge structure encompassing stretches rich in adenine nucleotides and a potential stem-loop domain appear to be the primary sites for protein binding. Removal of the 243-nucleotide segment results in a drastic reduction in protein binding and a concomitant increase in translational efficiency in vitro. We suggest that proteins binding to this region, including poly(A)-binding protein itself, may be essential for regulating translation of this mRNA.

Identification of two critical base pairings in 5' untranslated regions affecting translation efficiency of synthetic uncapped globin mRNAs

We observed a marked difference between the in vitro translation efficiency of two uncapped synthetic mRNAs, displaying the entire human alpha or beta globin mRNA sequences and some additional non-globin sequences in 5'. The comparison of the translation efficiencies of chimeric mRNAs indicated that the alpha 5' untranslated region (5' UTR) is responsible for a low translation efficiency that cannot be explained neither by primary sequence nor by the overall stability of 5' UTR secondary structures only. By point mutations in this alpha 5' UTR, we identified two base pairings at position -1 and -2 preceding the initiation codon which are associated with a negative effect on translation efficiency.

A consideration of alternative models for the initiation of translation in eukaryotes

Although recent biochemical and genetic investigations have produced some insights into the mechanism of initiation of translation in eukaryotic cells, two aspects of the initiation process remain controversial. One unsettled issue concerns a variety of functions that have been proposed for mRNA binding proteins, including some initiation factors. The need to distinguish between specific and nonspecific binding of proteins to mRNA is discussed herein. The possibility that certain initiation factors might act as RNA helicases is evaluated along with other ideas about the functions of mRNA- and ATP-binding factors. A second controversial issue concerns the universality of the scanning mechanism for initiation of translation. According to the conventional scanning model, the initial contact between eukaryotic ribosomes and mRNA occurs exclusively at the 5' terminus of the message, which is usually capped. The existence of uncapped mRNAs among a few plant and animal viruses has prompted a vigorous search for other modes of initiation. An "internal initiation" mechanism, first proposed for picornaviruses, has received considerable attention. Although a large body of evidence has been adduced in support of such a mechanism, many of the experiments appear flawed or inconclusive. Some suggestions are given for improving experiments designed to test the internal initiation hypothesis.

In vivo analysis of plant RNA structure: soybean 18S ribosomal and ribulose-1,5-bisphosphate carboxylase small subunit RNAs

A method to investigate the structure of RNA molecules within intact plant tissues has been developed. The RNA structures are analyzed using dimethyl sulfate (DMS), which modifies substituents of adenine and cytosine residues within single-stranded regions of RNA molecules. Reactive sites are identified by primer extension analysis. Using this procedure, an analysis of the secondary structure of the cytoplasmic 18S ribosomal RNA in soybean seedling leaves has been completed. DMS modification data are in good agreement with the phylogenetic structure predicted for soybean 18S rRNA. However, there are a few notable exceptions where residues thought to be involved in double-stranded regions in all 18S rRNAs are strongly modified in soybean leaf samples. These data taken together with the phylogenetic structure suggest that alternate structures may exist in vivo. The further applicability of this technique is demonstrated by comparing the modification pattern obtained in vivo to that obtained in vitro for a particular mRNA molecule encoding the small subunit of ribulose-1,5-bisphosphate carboxylase. The results obtained are compared to a predicted minimum energy secondary structure. The data indicate that the conformation of RNA molecules within the cell may not be reflected in a structural analysis of purified mRNA molecules.

'Phytoantibodies': a general vector for the expression of immunoglobulin domains in transgenic plants

Sequences encoding the immunoglobulin heavy-chain variable (VH) domains were engineered in a new general purpose vector to transform plants via Agrobacterium. The expression of an isolated VH domain (IVD) after introduction into the plant genome has been monitored by northern, western and immunohistochemical analysis. Immunoblotting showed that the polypeptide was stably expressed and accounted for up to 1% of the soluble protein fraction. It is therefore proposed that single immunoglobulin domains of suitable specificity expressed in plants may constitute an effective system to inhibit the activity of molecules involved in plant pathology or plant development.

Primary structure and expression of bovine poly(A) polymerase

Poly(A) polymerase has a critical role in the synthesis of messenger RNA in eukaryotic cells. The isolation and characterization of complementary DNAs encoding bovine poly(A) polymerase is described here. The predicted sequences of the mRNA and protein reveal features that provide insights into how the enzyme functions and how it might be regulated. Poly(A) polymerase expressed from a cloned cDNA is fully functional in in vitro assays, and mutational analyses have identified a putative regulatory domain that enhances, but is not essential for, activity.

Translational control of equine herpesvirus type 1 gene expression

Translational control mechanisms modulate gene expression in a variety of cellular and viral systems. Using hypertonic conditions to block protein synthesis in vivo, we observed that the synthesis of several major equine herpesvirus type 1 proteins was selectively inhibited. Although sensitivity to hypertonic conditions was graded across a continuum, messages coding for proteins of 203, 130.5, and 31.5 kDa were significantly more resistant to higher salt concentrations in vivo than those coding for polypeptides of 148, 116, and 74 kDa. Similar results were observed in vitro when potassium acetate was used to block initiation. In addition, Northern blot analyses demonstrated that steady-state levels of cellular mRNAs declined beginning at about 6 hr after infection. Taken together, these results indicate that the expression of several major equine herpesvirus type 1 genes was controlled in part at the post-transcriptional level.

Intracellular messengers and the control of protein synthesis

The molecular events responsible for controlling cell growth and development, as well as their coordinate interaction is only beginning to be revealed. At the basis of these controlling events are hormones, growth factors and mitogens which, through transmembrane signalling trigger an array of cellular responses, initiated by receptor-associated tyrosine kinases, which in turn either directly or indirectly mediate their effects through serine/threonine protein kinases. Utilizing the obligatory response of activation of protein synthesis in cell growth and development, we describe efforts to work backwards along the regulatory pathway to the receptor, identifying those molecular components involved in modulating the rate of translation. We begin by describing the components and steps of protein synthesis and then discuss in detail the regulatory pathways involved in the mitogenic response of eukaryotic cells and during meiotic maturation of oocytes. Finally we discuss possible future work which will further our understanding of these systems.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Translational efficiency: iridovirus early mRNAs outcompete tobacco mosaic virus message in vitro

Infection with the iridovirus, frog virus 3, results in the rapid inhibition of host cell protein synthesis and is correlated with activation of an eIF-2 kinase. Because phosphorylation of eIF-2 inhibits ternary complex formation and thus reduces the overall level of translation, it has been suggested that frog virus 3 messages escaped translational shut-off by outcompeting host messages for the remaining translational capacity of the cell. In this report, we show that frog virus 3 messages were more translationally competitive than highly efficient tobacco mosaic virus transcripts based on their relative resistance to inhibitors of initiation. This result strengthens the suggestion that the selective translation of frog virus 3 transcripts in virus-infected cells may be a reflection of their enhanced competitiveness.

The DNA binding subunit of NF-kappa B is identical to factor KBF1 and homologous to the rel oncogene product

The major determinant in the transcriptional control of class I genes of the major histocompatibility complex is an enhancer sequence located around -170 from the transcription start site, which binds a factor named KBF1. We have isolated a complementary cDNA coding for KBF1 and identified the DNA binding and dimerization domain of the protein. Because KBF1 and the transcription factor NF-kappa B bind to similar sequences, we investigated the relationship between these two molecules. It appeared that KBF1 is, by all criteria used, identical to the 50 kd DNA binding subunit of NF-kappa B. KBF1 (and therefore p50) also displays extensive amino acid sequence homology with the v-rel oncogene and the Drosophila maternal morphogen dorsal. In vitro experiments suggest functional homologies between KBF1 and v-rel.

Frog virus 3-mediated translational shut-off: frog virus 3 messages are translationally more efficient than host and heterologous viral messages under conditions of increased translational stress

Frog virus 3 rapidly and selectively blocks host cell translation while synthesizing more than 60 virus-specific polypeptides. Previous work indicated that virus infection led to activation of a kinase that phosphorylated and, as a consequence, inactivated eIF-2. Although phosphorylation of eIF-2 could explain the rapid decline in host cell translation, it could not explain how viral protein synthesis persisted in the face of host shut-off. To explain this phenomenon, we speculated that viral messages, either as a consequence of their higher translational efficiency or their greater abundance, were able to outcompete host messages for the remaining translational initiation complexes. To test this hypothesis, the relative translational efficiency of three characteristic FV3 messages was measured against that of several model messages. Translational efficiency was determined by monitoring the resistance (and hence the competitiveness) of a given transcript to increasing concentrations of salt in vitro and in vivo. In both rabbit reticulocyte lysates and wheat germ extracts, FV3 messages were more resistant to supra-optimal concentrations of potassium acetate than globin message and three BMV transcripts. In vivo, FV3 polypeptides were synthesized in the presence of salt concentrations that blocked host cell protein synthesis. These results suggest that the selective translation of FV3 messages in virus-infected cells may partly be due to the higher translational efficiency of viral messages. Structural features that contribute to translational efficiency are discussed.

Nucleotide sequence of rat S-adenosylmethionine decarboxylase cDNA. Comparison with an intronless rat pseudogene

Due to two different polyadenylation signals, two forms of S-adenosylmethionine decarboxylase (AdoMetDC) mRNA (2.1 and 3.4 kb) are present in human and rodent tissues. The nucleotide sequences of rat and human cDNAs corresponding to the shorter mRNA were published previously by us [Pajunen et al., J. Biol. Chem. 263 (1988) 17040-17049]. These sequences covered the coding regions but were incomplete at their 5' ends. Here we report the sequence of rat cDNA spanning the entire longer mRNA with a substantially extended leader region, and compare the sequence with that of a rat psi AdoMetDC pseudogene isolated from a rat genomic library. Relative to the mRNA, the pseudogene has multiple base changes as well as insertions, and deletions. Furthermore, it lacks introns, and is flanked by a short direct repeat. These are typical characteristics of a processed retrogene.

Selection of the 5'-proximal translation initiation site is influenced by mRNA and eIF-2 concentrations

A cDNA clone of the influenza virus NS (non-structural protein) gene in a vector carrying a bacteriophage T7 RNA polymerase promoter was manipulated so as to reiterate the initiation site to give two in-frame AUG codons 57 nucleotide residues apart. Each initiation site was in either a preferred context (...AUAAUGG...) or a less favourable context (...UUUAUGG...) and the four possible permutations were constructed. When capped mRNA transcripts of these clones were translated in the rabbit reticulocyte lysate system, products from initiation at both AUG codons were observed. At low RNA concentrations the frequency of initiation at the 5'-proximal AUG codon rather than the second was higher when the first AUG codon was in the preferred context, in qualitative agreement with the scanning ribosome model. However, a completely unexpected finding was that the ratio of initiation at the first AUG codon to initiation at the second decreased with increasing mRNA concentration, irrespective of the particular context involved. Several lines of evidence indicated that the increased frequency of initiation at the second AUG codon was not due solely to the lower density of ribosome loading per mRNA at high RNA concentrations, and may therefore be the result of high RNA concentrations out-titring the capacity of endogenous reticulocyte factors responsible for preferential initiation at the 5'-proximal AUG codon. The effect of supplementing the system with purified initiation factors was examined. Only eIF-2 was capable of decreasing the frequency of initiation at the second AUG codon and promoting use of the first AUG at high mRNA concentrations; eIF-3, 4A, 4B, 4C + 4D, 4F and 5 were inactive.

DNA sequence and comparative analyses of the equine herpesvirus type 1 immediate early gene

The immediate early (IE) proteins of herpesviruses are important regulatory factors which control the expression of genes at the transcriptional level. We report the DNA sequence of the immediate early gene of the alphaherpesvirus equine herpesvirus type 1 (EHV-1). This sequence is shown to be extremely rich in guanine and cytosine, resulting in a highly biased codon usage. The IE gene region possesses 38 open reading frames (ORFs) greater than 300 bp in length, 11 of which have coding regions of at least 100 amino acids (aa) following potential translation initiator codons. The largest ORF consists of 1487 codons (4461 bp) starting with the first ATG and would encode a protein of MW 155,000. TATA and CCAAT sequences as well as several potential cis-acting elements lie upstream to the major ORF. The deduced amino acid sequence for the 155,000 protein has a high degree of homology to the herpes simplex virus type 1 (HSV-1) ICP4 protein and its varicella-zoster virus (VZV) homolog. The regions of the EHV-1 IE protein that are homologous with these proteins correspond to the previously determined pattern of homology between the HSV and VZV IE polypeptides. However, there are are a number of differences within these broadly defined regions. It is therefore expected that this comparative study will facilitate the identification of functionally important residues within the amino acid sequence of IE proteins.

Differential inhibition of downstream gene expression by the cauliflower mosaic virus 35S RNA leader

The effect of the 600 nucleotide-long CaMV 35S RNA 5' leader sequence on the expression of downstream genes was analyzed both in plant protoplasts and in vitro. For transient expression studies in protoplasts derived from host and nonhost plants, the bacterial chloramphenicol acetyl transferase (CAT) gene was fused to the initiation codon of ORF VII. The leader sequence reduced CAT expression two- to four-fold in protoplasts derived from three host species, but 10- to 50-fold in protoplasts derived from three different nonhost species. For in-vitro studies the 35S promoter was replaced by the SP6 promoter. The leader reduced in-vitro translation of SP6 transcripts approximately six-fold, indicating that at least part of the inhibition observed in protoplasts is directly due to the interference of the leader sequence with translation. Other steps in gene expression that may also be affected are discussed.

The Neurospora clock gene frequency shares a sequence element with the Drosophila clock gene period

The isolation and characterization of single gene mutations affecting the circadian biological clocks of several organisms has left little doubt that circadian rhythms can be subjected to classical genetical analysis. Many of these mutations occur at the same few genetic loci (frequency (frq) in the fungus Neurospora, and period (per) in fruit fly Drosophila); these loci represent the best studied clock-affecting genes known. Mutant strains are usually affected in more than one basic clock property, suggesting an inter-relatedness at the molecular level among these basic properties that would not have been predicted a priori. The extensive background information available concerning the frq locus provides a basis for the molecular dissection of the Neurospora circadian clock--the most minimal circadian system thus far described. We report here the cloning and analysis of the frq locus and show it to be larger and more complex than would have been predicted from the available genetic data. Complete rescue of all of the pleiotropic mutant phenotypes of the recessive frq allele requires transformation with a 7.7-kilobase (kb) region of DNA encoding at least two transcripts. Sequence analysis of this region has allowed the identification of a common element between frq and per which, given the background similarities in their classical genetic characteristics, suggests the possibility of a common element in the clock mechanisms of these two organisms.

Inhibition of heat shock (stress) protein induction by deuterium oxide and glycerol: additional support for the abnormal protein hypothesis of induction

The patterns of radioactively labeled proteins from cultured chicken embryo cells stressed in the presence of either D2O or glycerol were analyzed by using one-dimensional polyacrylamide gel electrophoresis. These hyperthermic protectors blocked the induction of stress proteins during a 1-hour heat shock at 44 degrees C. The inhibitory effect of glycerol but not D2O on the induction of heat shock proteins could be overcome by increased temperature. By using transcriptional run-on assays of isolated nuclei and cDNA probes to detect hsp70- and hsp88-specific RNA transcripts, it was shown that the D2O and glycerol blocks occurred at or before transcriptional activation of the hsp70 and hsp88 genes. After heat-stressed cells were returned to 37 degrees C and the protectors were removed, heat shock proteins were inducible by a second heating. This result and the fact that the chemical stressor sodium arsenite induced stress proteins in glycerol medium indicated that the treatments did not irreversibly inhibit the induction pathways and that the stress response could be triggered even in the presence of glycerol by a stressor other than heat. In principle then, cells incurring thermal damage during a 1-hour heat shock at 44 degrees C in D2O or glycerol medium should be competent to respond by inducing heat shock proteins during a subsequent recovery period at 37 degrees C in normal medium. We found that heat shock proteins were not induced in recovering cells, suggesting that glycerol and D2O protected heat-sensitive targets from thermal damage. Evidence that the heat-sensitive target(s) is likely to be a protein(s) is summarized. During heat shocks of up to 3 hours duration, neither D2O nor glycerol significantly altered hsp23 gene activity, a constitutively expressed chicken heat shock gene whose RNA transcripts and protein products are induced by stabilization (increased half-life). During a 2-hour heat shock, glycerol treatment blocked the heat-induced stabilization of hsp23 RNA and proteins; however, D2O treatment only blocked RNA transcript stabilization, effectively uncoupling the hsp23 protein stabilization pathway from hsp23 RNA stabilization and transcriptional activation of hsp70 and hsp88 genes.

Visualizing mRNA expression in plant protoplasts: factors influencing efficient mRNA uptake and translation

In this paper we demonstrate that RNA sequences present upstream and downstream of a reporter gene coding region play an important role in determining the amount of protein produced from an mRNA. A translational enhancer, omega, derived from tobacco mosaic virus, when present at the 5'-end of beta-glucuronidase mRNA increased the efficiency of translation 16-fold to 18-fold in electroporated tobacco or carrot protoplasts, and threefold to 11-fold in maize or rice protoplasts. The presence of omega did not alter the half-life of the mRNA in vivo. We also demonstrate for the first time that a minimum polyadenylated tail length of 25 adenylate residues is sufficient to substantially increase the expression and half-life of the reporter mRNA in plants. When in vitro-produced mRNAs were synthesized such that extra sequence was added to the 3'-end of the poly(A) tail, however, the final level of expression was decreased up to 80%. Omega, the translational enhancer, and a poly(A) tail function independently of each other; their combined effect on translation, when both are present in an mRNA, is the multiplication of their individual effects. Histochemical analysis for the presence of beta-glucuronidase in tobacco established that virtually all viable cells receive mRNA during electroporation. Video image analysis of tobacco protoplasts electroporated with luciferase mRNA demonstrated that there is a wide range in the level of expression of this marker. Carrier RNA, when present during electroporation, had only a modest effect on increasing mRNA uptake. Reporter mRNA expression in electroporated protoplasts was directly proportional to the input mRNA up to at least 30 micrograms/ml.