Multiple cis-acting elements modulate the translational efficiency of GCN4 mRNA in yeast

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Expression and characterization of splice variants of PYK2, a focal adhesion kinase-related protein

Focal adhesion kinase and the recently identified proline-rich tyrosine kinase 2 (PYK2), also known as cell adhesion kinase β, related adhesion focal tyrosine kinase or calcium-dependent protein tyrosine kinase, define a new family of non-receptor protein tyrosine kinases. Activation of PYK2 has been implicated in multiple signaling events, including modulation of ion channels, T- and B-cell receptor signaling and cell death. Mechanisms underlying the functional diversity of PYK2 are unclear. Here, we provide evidence for two novel alternatively expressed isoforms of PYK2. One isoform, designated PYK2s (PYK2 splice form), appears to be a splice variant of PYK2 lacking 42 amino acids within the C-terminal domain. A second isoform, referred to as PRNK (PYK2-related non-kinase), appears to be specified by mRNAs that encode only part of the C-terminal domain of PYK2. Northern blot analysis indicates that the unspliced PYK2 is expressed at high levels in the brain and poorly expressed in the spleen, whereas PYK2s and PRNK are expressed in the spleen. In situ hybridization studies of rat brain demonstrate that the unspliced PYK2 is selectively expressed at high levels in hippocampus, cerebral cortex and olfactory bulb, whereas PYK2s and PRNK are expressed at low levels in all regions of rat brain examined. Immunofluorescence analysis of ectopically expressed PRNK protein shows that PRNK, in contrast to full-length PYK2, is localized to focal adhesions by sequences within the focal adhesion targeting domain. In addition, PYK2, but not PRNK, interacts with p130(cas)and Graf. These results imply that PRNK may selectively regulate PYK2 function in certain cells by binding to some but not all PYK2 binding partners, and the functional diversity mediated by PYK2 may be due in part to complex alternative splicing.

Genetic evidence for functional specificity of the yeast GCN2 kinase

In yeast the GCN2 kinase mediates translational control of GCN4 by phosphorylating the alpha subunit of eIF-2 in response to extracellular amino acid limitation. Although phosphorylation of eIF-2 alpha has been shown to inhibit global protein synthesis, amino acid starvation results in a specific activation effect on GCN4 mRNA translation. Under the same conditions, translation of other mRNAs appears only slightly affected. The mechanism responsible for the observed selectivity of the GCN2 kinase is not clear. Here, we present genetic evidence that suggests that locally restricted action of the GCN2 kinase facilitates GCN4-specific translational regulation.

Translational efficiency is regulated by the length of the 3' untranslated region

All polyadenylated mRNAs contain sequence of variable length between the coding region and the poly(A) tail. Little has been done to establish what role the length of the 3' untranslated region (3'UTR) plays in posttranscriptional regulation. Using firefly luciferase (luc) reporter mRNA in transiently transfected Chinese hamster ovary (CHO) cells, we observed that the addition of a poly(A) tail increased expression 97-fold when the length of the 3'UTR was 19 bases but that its stimulatory effect was only 2.3-fold when the length of the 3'UTR was increased to 156 bases. The effect of the luc 3'UTR on poly(A) tail function was orientation independent, suggesting that its length and not its primary sequence was the important factor. Increasing the length of the 3'UTR increased expression from poly(A)- mRNA but had little effect on poly(A)+ mRNA. To examine the effect of length on translational efficiency and mRNA stability, a 20-base sequence was introduced and reiterated downstream of the luc stop codon to generate a nested set of constructs in which the length of the 3'UTR increased from 4 to 104 bases. For poly(A)- reporter mRNA, translational efficiency in CHO cells increased 38-fold as the length of the 3'UTR increased from 4 to 104 bases. Increasing the length of the 3'UTR beyond 104 bases increased expression even further. Increasing the length of the 3'UTR also resulted in a 2.5-fold stabilization of the reporter mRNA. For poly(A)+ mRNA, the translational efficiency and mRNA half-life increased only marginally as the length of the 3'UTR increased from 27 to 161 bases. However, positioning the poly(A) tail only 7 bases downstream of the stop codon resulted in a 39-fold reduction in the rate of translation relative to a construct with a 27-base 3'UTR, which may be a consequence of the poly(A) tail-poly(A)-binding protein complex functioning as a steric block to translocating ribosomes as they approached the termination codon. The optimal length of the 3' noncoding region for maximal poly(A) tail-mediated stimulation of translation is approximately 27 bases. These data suggest that the length of the 3'UTR plays an important role in determining both the translational efficiency and the stability of an mRNA.

A transient GCN4 mRNA destabilization follows GCN4 translational derepression

Studies based on experimental strategies that utilized either inhibitors or structural alterations point to the existence of an inverse relationship between translation and stability of a given mRNA. In this study we have investigated the potential link between translation and stability of the yeast GCN4 mRNA whose translational rates change with respect to amino acid availability. We observed that under conditions favoring its translation, the steady state levels of the GCN4 mRNA were decreased, but this was not due to a measurable alternation in its decay rate. We have demonstrated that an extensive destabilization of this message is intimately coupled with its increased access to heavy polysomes, which occurs transiently in the process of translational derepression. This transient change in the stability is what readjusts the steady state levels of the GCN4 mRNA. This study demonstrates in vivo the existence of a mechanism of mRNA degradation that is coupled with the process of translation.

Transcriptional interference caused by GCN4 overexpression reveals multiple interactions mediating transcriptional activation

Overproduction of Gcn4p in yeast cells resulted in the inhibition of transcription from promoters controlled by the GAL4 or dA:dT elements. We have demonstrated that this effect is mediated through the activation domain of Gcn4p and that the function of the transcriptional activator at the affected promoter is impaired. The inhibitory effect of Gcn4p and that the function of the transcriptional activator at the affected promoter is impaired. The inhibitory effect of Gcn4p on these promoters persisted in yeast strains disrupted for the ADA2 and/or GCN5 genes, whose products are required for only part of the transcriptional activation capacity of Gcn4p and other activators, but was alleviated by overexpression of gamma TFIIB. These results support the hypothesis that general transcription factors become unavailable at certain promoters when an activator is overexpressed and strongly imply the existence of an Ada2p/Gcn5p-independent pathway of communication between acidic activators and the basic transcription machinery. In a genetic screen, we have isolated a mutation which neutralises the squelching effects of Gcn4p. This AFR1-1 (activation function reduced) mutation is dominant, it affects the transcriptional activation properties of a number of activators and results in lethality when combined with a gcn5 disruption. Our results suggest that the AFR1 gene product is involved in the mediation of transcriptional activation.

The role of the 5' untranslated region of eukaryotic messenger RNAs in translation and its investigation using antisense technologies

This chapter discusses the recent advances in the field of translational control and the possibility of applying the powerful antisense technology to investigate some of the unanswered questions, especially those pertaining to the role of the 5’untranslated region ( UTR) on translation initiation. Translational regulation is predominantly exerted during the initiation phase that is considered to be the rate-limiting step. Two types of translational regulation can be distinguished: global, in which the initiation rate of (nearly) all cellular messenger RNA (mRNA) is controlled and selective, in which the translation rate of specific mRNAs varies in response to the biological stimuli. In most cases of global regulation, control is exerted via the phosphorylation state of certain initiation factors, whereas only a few examples of selective regulation have been characterized well enough to define the underlying molecular events. Interestingly, cis-acting regulatory sequences, affecting translation initiation, have been found not only in the 5’UTRs of selectively regulated mRNAs, but also in the 3’UTRs. Thus, in addition to the protein encoding open reading frames, both the 5’ and 3’UTRs of mRNAs must be considered for their effect on translation.

Functional expression of the transcriptional activator Opaque-2 of Zea mays in transformed yeast

The aim of this research was to determine whether the structural homology between the O2 gene, a maize transcriptional activator, and the GCN4 gene, a yeast transcriptional factor, is reflected at the level of function. The O2 cDNA was cloned in the yeast expression vector pEMBLyex4 under the control of a hybrid inducible promoter, and used to transform the yeast Saccharomyces cerevisiae. Transformed yeast cells produced O2 mRNA and a polypeptide immunoreactive with anti-O2 antibodies during growth in galactose. The heterologous protein was correctly translocated into the yeast nuclei, as demonstrated by immunofluorescence, indicating that the nuclear targeting sequences of maize are recognized by yeast cells. Further experiments demonstrated the ability of O2 to rescue a gcn4 mutant grown in the presence of aminotriazole, an inhibitor of the HIS3 gene product, suggesting that O2 activates the HIS3 gene, gene normally under control of GCN4. It was shown that the O2 protein is able to trans-activate the HIS4 promoter in yeast cells and binds to it in vitro. The sequence protected by O2, TGACTC, is also the binding site for GCN4. Finally, the expression of O2 protein in yeast did not produce alterations during batch growth at 30 degrees C, while transformants expressing O2 protein showed a conditionally lethal phenotype when grown in galactose at 36 degrees C; this phenotype mimics the behaviour of gcd mutants. The results support the idea that basic mechanisms of transcription control have been highly conserved in eukaryotes.

Gene-specific translational control of the yeast GCN4 gene by phosphorylation of eukaryotic initiation factor 2

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) is one of the best-characterized mechanisms for down-regulating total protein synthesis in mammalian cells in response to various stress conditions. Recent work indicates that regulation of the GCN4 gene of Saccharomyces cerevisiae by amino acid availability represents a gene-specific case of translational control by phosphorylation of eIF-2 alpha. Four short open reading frames in the leader of GCN4 mRNA (uORFs) restrict the flow of scanning ribosomes from the cap site to the GCN4 initiation codon. When amino acids are abundant, ribosomes translate the first uORF and reinitiate at one of the remaining uORFs in the leader, after which they dissociate from the mRNA. Under conditions of amino acid starvation, many ribosomes which have translated uORF1 fail to reinitiate at uORFs 2-4 and utilize the GCN4 start codon instead. Failure to reinitiate at uORFs 2-4 in starved cells results from a reduction in the GTP-bound form of eIF-2 that delivers charged initiator tRNA(iMet) to the ribosome. When the levels of eIF-2.GTP.Met-tRNA(iMet) ternary complexes are low, many ribosomes will not rebind this critical initiation factor following translation of uORF1 until after scanning past uORF4, but before reaching GCN4. Phosphorylation of eIF-2 by the protein kinase GCN2 decreases the concentration of eIF-2.GTP.Met-tRNA(iMet) complexes by inhibiting the guanine nucleotide exchange factor for eIF-2, which is the same mechanism utilized in mammalian cells to inhibit total protein synthesis by phosphorylation of eIF-2.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

Mechanism of translation of monocistronic and multicistronic human immunodeficiency virus type 1 mRNAs

We have used a panel of cDNA clones expressing wild-type and mutant human immunodeficiency virus type 1 (HIV-1) mRNAs to study translation of these mRNAs in eucaryotic cells. The tat open reading frame (ORF) has a strong signal for translation initiation, while rev and vpu ORFs have weaker signals. The expression of downstream ORFs is inhibited in mRNAs that contain the tat ORF as the first ORF. In contrast, downstream ORFs are expressed efficiently from mRNAs that have rev or vpu as the first ORF. All env mRNAs contain the upstream vpu ORF. Expression of HIV-1 Env protein requires a weak vpu AUG, which allows leaky scanning to occur, thereby allowing ribosomes access to the downstream env ORF. We concluded that HIV-1 mRNAs are translated by the scanning mechanism and that expression of more than one protein from each mRNA was caused by leaky scanning at the first AUG of the mRNA.

Mechanism of translation of monocistronic and multicistronic human immunodeficiency virus type 1 mRNAs

We have used a panel of cDNA clones expressing wild-type and mutant human immunodeficiency virus type 1 (HIV-1) mRNAs to study translation of these mRNAs in eucaryotic cells. The tat open reading frame (ORF) has a strong signal for translation initiation, while rev and vpu ORFs have weaker signals. The expression of downstream ORFs is inhibited in mRNAs that contain the tat ORF as the first ORF. In contrast, downstream ORFs are expressed efficiently from mRNAs that have rev or vpu as the first ORF. All env mRNAs contain the upstream vpu ORF. Expression of HIV-1 Env protein requires a weak vpu AUG, which allows leaky scanning to occur, thereby allowing ribosomes access to the downstream env ORF. We concluded that HIV-1 mRNAs are translated by the scanning mechanism and that expression of more than one protein from each mRNA was caused by leaky scanning at the first AUG of the mRNA.

Aromatic amino acid biosynthesis in the yeast Saccharomyces cerevisiae: a model system for the regulation of a eukaryotic biosynthetic pathway

This review focuses on the gene-enzyme relationships and the regulation of different levels of the aromatic amino acid biosynthetic pathway in a simple eukaryotic system, the unicellular yeast Saccharomyces cerevisiae. Most reactions of this branched pathway are common to all organisms which are able to synthesize tryptophan, phenylalanine, and tyrosine. The current knowledge about the two main control mechanisms of the yeast aromatic amino acid biosynthesis is reviewed. (i) At the transcriptional level, most structural genes are regulated by the transcriptional activator GCN4, the regulator of the general amino acid control network, which couples transcriptional derepression to amino acid starvation of numerous structural genes in multiple amino acid biosynthetic pathways. (ii) At the enzyme level, the carbon flow is controlled mainly by modulating the enzyme activities at the first step of the pathway and at the branch points by feedback action of the three aromatic amino acid end products. Implications of these findings for the relationship of S. cerevisiae to prokaryotic as well as to higher eukaryotic organisms and for general regulatory mechanisms occurring in a living cell such as initiation of transcription, enzyme regulation, and the regulation of a metabolic branch point are discussed.

Complex formation by positive and negative translational regulators of GCN4

GCN4 is a transcriptional activator of amino acid biosynthetic genes in Saccharomyces cerevisiae whose expression is regulated by amino-acid availability at the translational level. GCD1 and GCD2 are negative regulators required for the repression of GCN4 translation under nonstarvation conditions that is mediated by upstream open reading frames (uORFs) in the leader of GCN4 mRNA. GCD factors are thought to be antagonized by the positive regulators GCN1, GCN2 and GCN3 in amino acid-starved cells to allow for increased GCN4 protein synthesis. Previous genetic studies suggested that GCD1, GCD2, and GCN3 have closely related functions in the regulation of GCN4 expression that involve translation initiation factor 2 (eIF-2). In agreement with these predictions, we show that GCD1, GCD2, and GCN3 are integral components of a high-molecular-weight complex of approximately 600,000 Da. The three proteins copurified through several biochemical fractionation steps and could be coimmunoprecipitated by using antibodies against GCD1 or GCD2. Interestingly, a portion of the eIF-2 present in cell extracts also cofractionated and coimmunoprecipitated with these regulatory proteins but was dissociated from the GCD1/GCD2/GCN3 complex by 0.5 M KCl. Incubation of a temperature-sensitive gcdl-101 mutant at the restrictive temperature led to a rapid reduction in the average size and quantity of polysomes, plus an accumulation of inactive 80S ribosomal couples; in addition, excess amounts of eIF-2 alpha, GCD1, GCD2, and GCN3 were found comigrating with free 40S ribosomal subunits. These results suggest that GCD1 is required for an essential function involving eIF-2 at a late step in the translation initiation cycle. We propose that lowering the function of this high-molecular-weight complex, or of eIF-2 itself, in amino acid-starved cells leads to reduced ribosomal recognition of the uORFs and increased translation initiation at the GCN4 start codon. Our results provide new insights into how general initiation factors can be regulated to affect gene-specific translational control.

Ribosome association of GCN2 protein kinase, a translational activator of the GCN4 gene of Saccharomyces cerevisiae

The GCN4 gene of the yeast Saccharomyces cerevisiae encodes a transcriptional activator of amino acid biosynthetic genes that is regulated at the translational level according to the availability of amino acids. GCN2 is a protein kinase required for increased translation of GCN4 mRNA in amino acid-starved cells. Centrifugation of cell extracts in sucrose gradients indicated that GCN2 comigrates with ribosomal subunits and polysomes. The fraction of GCN2 cosedimenting with polysomes was reduced under conditions in which polysomes were dissociated, suggesting that GCN2 is physically bound to these structures. When the association of 40S and 60S subunits was prevented by omitting Mg2+ from the gradient, almost all of the GCN2 comigrated with 60S ribosomal subunits, and it remained bound to these particles during gel electrophoresis under nondenaturing conditions. GCN2 could be dissociated from 60S subunits by 0.5 M KCl, suggesting that it is loosely associated with ribosomes rather than being an integral ribosomal protein. Accumulation of GCN2 on free 43S-48S particles and 60S subunits occurred during polysome runoff in vitro and under conditions of reduced growth rate in vivo. These observations, plus the fact that GCN2 shows preferential association with free ribosomal subunits during exponential growth, suggest that GCN2 interacts with ribosomes during the translation initiation cycle. The extreme carboxyl-terminal segment of GCN2 is essential for its interaction with ribosomes. These sequences are also required for the ability of GCN2 to stimulate GCN4 translation in vivo, leading us to propose that ribosome association by GCN2 is important for its access to substrates in the translational machinery or for detecting uncharged tRNA in amino acid-starved cells.

Ribosome association of GCN2 protein kinase, a translational activator of the GCN4 gene of Saccharomyces cerevisiae

The GCN4 gene of the yeast Saccharomyces cerevisiae encodes a transcriptional activator of amino acid biosynthetic genes that is regulated at the translational level according to the availability of amino acids. GCN2 is a protein kinase required for increased translation of GCN4 mRNA in amino acid-starved cells. Centrifugation of cell extracts in sucrose gradients indicated that GCN2 comigrates with ribosomal subunits and polysomes. The fraction of GCN2 cosedimenting with polysomes was reduced under conditions in which polysomes were dissociated, suggesting that GCN2 is physically bound to these structures. When the association of 40S and 60S subunits was prevented by omitting Mg2+ from the gradient, almost all of the GCN2 comigrated with 60S ribosomal subunits, and it remained bound to these particles during gel electrophoresis under nondenaturing conditions. GCN2 could be dissociated from 60S subunits by 0.5 M KCl, suggesting that it is loosely associated with ribosomes rather than being an integral ribosomal protein. Accumulation of GCN2 on free 43S-48S particles and 60S subunits occurred during polysome runoff in vitro and under conditions of reduced growth rate in vivo. These observations, plus the fact that GCN2 shows preferential association with free ribosomal subunits during exponential growth, suggest that GCN2 interacts with ribosomes during the translation initiation cycle. The extreme carboxyl-terminal segment of GCN2 is essential for its interaction with ribosomes. These sequences are also required for the ability of GCN2 to stimulate GCN4 translation in vivo, leading us to propose that ribosome association by GCN2 is important for its access to substrates in the translational machinery or for detecting uncharged tRNA in amino acid-starved cells.

Complex formation by positive and negative translational regulators of GCN4

GCN4 is a transcriptional activator of amino acid biosynthetic genes in Saccharomyces cerevisiae whose expression is regulated by amino-acid availability at the translational level. GCD1 and GCD2 are negative regulators required for the repression of GCN4 translation under nonstarvation conditions that is mediated by upstream open reading frames (uORFs) in the leader of GCN4 mRNA. GCD factors are thought to be antagonized by the positive regulators GCN1, GCN2 and GCN3 in amino acid-starved cells to allow for increased GCN4 protein synthesis. Previous genetic studies suggested that GCD1, GCD2, and GCN3 have closely related functions in the regulation of GCN4 expression that involve translation initiation factor 2 (eIF-2). In agreement with these predictions, we show that GCD1, GCD2, and GCN3 are integral components of a high-molecular-weight complex of approximately 600,000 Da. The three proteins copurified through several biochemical fractionation steps and could be coimmunoprecipitated by using antibodies against GCD1 or GCD2. Interestingly, a portion of the eIF-2 present in cell extracts also cofractionated and coimmunoprecipitated with these regulatory proteins but was dissociated from the GCD1/GCD2/GCN3 complex by 0.5 M KCl. Incubation of a temperature-sensitive gcdl-101 mutant at the restrictive temperature led to a rapid reduction in the average size and quantity of polysomes, plus an accumulation of inactive 80S ribosomal couples; in addition, excess amounts of eIF-2 alpha, GCD1, GCD2, and GCN3 were found comigrating with free 40S ribosomal subunits. These results suggest that GCD1 is required for an essential function involving eIF-2 at a late step in the translation initiation cycle. We propose that lowering the function of this high-molecular-weight complex, or of eIF-2 itself, in amino acid-starved cells leads to reduced ribosomal recognition of the uORFs and increased translation initiation at the GCN4 start codon. Our results provide new insights into how general initiation factors can be regulated to affect gene-specific translational control.

Biosynthesis of human fibroblast growth factor-5

We have analyzed the biosynthesis of human fibroblast growth factor-5 (FGF-5) at the translational and posttranslational levels. FGF-5 RNA synthesized in vitro can be translated in rabbit reticulocyte lysates to yield a 29,500-Da protein, which is consistent with the molecular weight predicted from the coding sequence. The efficiency of FGF-5 translation is dramatically enhanced if an upstream open reading frame (ORF-1) in the RNA is deleted or if both AUG codons in ORF-1 are destroyed by point mutations, while partial enhancement is achieved by individual mutation of either ORF-1 AUG codon. These data suggest that FGF-5 synthesis requires the scanning of ribosomes past the two ORF-1 AUG codons. The introduction of these ORF-1 mutations into a eukaryotic FGF-5 expression vector increases its capacity to transform mouse NIH 3T3 cells up to 50-fold upon transfection. FGF-5 is secreted from transfected 3T3 cells and from human tumor cells as glycoproteins containing heterogeneous amounts of sialic acid. Glycosidase treatments suggest that the growth factor bears both N-linked and O-linked sugars.

Biosynthesis of human fibroblast growth factor-5

We have analyzed the biosynthesis of human fibroblast growth factor-5 (FGF-5) at the translational and posttranslational levels. FGF-5 RNA synthesized in vitro can be translated in rabbit reticulocyte lysates to yield a 29,500-Da protein, which is consistent with the molecular weight predicted from the coding sequence. The efficiency of FGF-5 translation is dramatically enhanced if an upstream open reading frame (ORF-1) in the RNA is deleted or if both AUG codons in ORF-1 are destroyed by point mutations, while partial enhancement is achieved by individual mutation of either ORF-1 AUG codon. These data suggest that FGF-5 synthesis requires the scanning of ribosomes past the two ORF-1 AUG codons. The introduction of these ORF-1 mutations into a eukaryotic FGF-5 expression vector increases its capacity to transform mouse NIH 3T3 cells up to 50-fold upon transfection. FGF-5 is secreted from transfected 3T3 cells and from human tumor cells as glycoproteins containing heterogeneous amounts of sialic acid. Glycosidase treatments suggest that the growth factor bears both N-linked and O-linked sugars.

Suppression of ribosomal reinitiation at upstream open reading frames in amino acid-starved cells forms the basis for GCN4 translational control

GCN4 encodes a transcriptional activator of amino acid-biosynthetic genes in Saccharomyces cerevisiae that is regulated at the translational level by upstream open reading frames (uORFs) in its mRNA leader. uORF4 (counting from the 5' end) is sufficient to repress GCN4 under nonstarvation conditions; uORF1 is required to overcome the inhibitory effect of uORF4 and stimulate GCN4 translation in amino acid-starved cells. Insertions of sequences with the potential to form secondary structure around uORF4 abolish derepression, indicating that ribosomes reach GCN4 by traversing uORF4 sequences rather than by binding internally to the GCN4 start site. By showing that wild-type regulation occurred even when uORF4 was elongated to overlap GCN4 by 130 nucleotides, we provide strong evidence that those ribosomes which translate GCN4 do so by ignoring the uORF4 AUG start codon. This conclusion is in accord with the fact that translation of a uORF4-lacZ fusion was lower in a derepressed gcd1 mutant than in a nonderepressible gcn2 strain. We also show that increasing the distance between uORF1 and uORF4 to the wild-type spacing that separates uORF1 from GCN4 specifically impaired the ability of uORF1 to derepress GCN4 translation. As expected, this alteration led to increased uORF4-lacZ translation in gcd1 cells. Our results suggest that under starvation conditions, a substantial fraction of ribosomes that translate uORF1 fail to reassemble the factors needed for reinitiation by the time they scan to uORF4, but become competent to reinitiate after scanning the additional sequences to GCN4. Under nonstarvation conditions, ribosomes would recover more rapidly from uORF1 translation, causing them all to reinitiate at uORF4 rather than at GCN4.

Suppression of ribosomal reinitiation at upstream open reading frames in amino acid-starved cells forms the basis for GCN4 translational control

GCN4 encodes a transcriptional activator of amino acid-biosynthetic genes in Saccharomyces cerevisiae that is regulated at the translational level by upstream open reading frames (uORFs) in its mRNA leader. uORF4 (counting from the 5' end) is sufficient to repress GCN4 under nonstarvation conditions; uORF1 is required to overcome the inhibitory effect of uORF4 and stimulate GCN4 translation in amino acid-starved cells. Insertions of sequences with the potential to form secondary structure around uORF4 abolish derepression, indicating that ribosomes reach GCN4 by traversing uORF4 sequences rather than by binding internally to the GCN4 start site. By showing that wild-type regulation occurred even when uORF4 was elongated to overlap GCN4 by 130 nucleotides, we provide strong evidence that those ribosomes which translate GCN4 do so by ignoring the uORF4 AUG start codon. This conclusion is in accord with the fact that translation of a uORF4-lacZ fusion was lower in a derepressed gcd1 mutant than in a nonderepressible gcn2 strain. We also show that increasing the distance between uORF1 and uORF4 to the wild-type spacing that separates uORF1 from GCN4 specifically impaired the ability of uORF1 to derepress GCN4 translation. As expected, this alteration led to increased uORF4-lacZ translation in gcd1 cells. Our results suggest that under starvation conditions, a substantial fraction of ribosomes that translate uORF1 fail to reassemble the factors needed for reinitiation by the time they scan to uORF4, but become competent to reinitiate after scanning the additional sequences to GCN4. Under nonstarvation conditions, ribosomes would recover more rapidly from uORF1 translation, causing them all to reinitiate at uORF4 rather than at GCN4.

Translational activation of GCN4 mRNA in a cell-free system is triggered by uncharged tRNAs

Translation of GCN4 mRNA is activated when yeast cells are grown under conditions of amino acid limitation. In this study, we established the conditions through which translation of the GCN4 mRNA could be activated in a homologous in vitro system. This activation paralleled the in vivo situation: it required the small open reading frames located in the 5' untranslated region of the GCN4 mRNA, and it was coupled with reduced rates of 43S preinitiation complex formation. Translational derepression in vitro was triggered by uncharged tRNA molecules, demonstrating that deacylated tRNAs are more proximal signals for translational activation of the GCN4 mRNA.

Translational activation of GCN4 mRNA in a cell-free system is triggered by uncharged tRNAs

Translation of GCN4 mRNA is activated when yeast cells are grown under conditions of amino acid limitation. In this study, we established the conditions through which translation of the GCN4 mRNA could be activated in a homologous in vitro system. This activation paralleled the in vivo situation: it required the small open reading frames located in the 5' untranslated region of the GCN4 mRNA, and it was coupled with reduced rates of 43S preinitiation complex formation. Translational derepression in vitro was triggered by uncharged tRNA molecules, demonstrating that deacylated tRNAs are more proximal signals for translational activation of the GCN4 mRNA.

Human arylamine N-acetyltransferase genes: isolation, chromosomal localization, and functional expression

N-Acetylation by hepatic arylamine N-acetyltransferase (NAT, EC 2.3.1.5) is a major route in the metabolism and detoxification of numerous drugs and foreign chemicals. NAT is the target of a common genetic polymorphism of clinical relevance in human populations. We have used our recently isolated rabbit cDNA rnat to clone three human NAT genes from human leukocyte DNA. None of the three genomic coding sequences was interrupted by introns. Two genes, designated NAT1 and NAT2, each possessed open reading frames of 870 bp. Both genes have been assigned to human chromosome 8, pter-q11. Following transfection they were transiently expressed in monkey kidney COS-1 cells. NAT1 and NAT2 gave rise to functional NAT proteins, as judged by their NAT enzyme activity with the arylamine substrate sulfamethazine. Western blots with NAT-specific antisera detected proteins of apparent molecular weight of 33 and 31 kD in NAT1- and NAT2-transfected cultures, respectively. The product of NAT2 had an identical apparent molecular weight as that of NAT detected in human liver cytosol. The deduced amino acid sequence of NAT2 also contained 6 peptide sequences which had previously been determined from tryptic peptides of the polymorphic NAT purified from human liver. These data suggest that NAT2 encodes the polymorphic NAT protein. The third gene, NATP, had multiple deleterious mutations and did not encode a functional NAT protein; it most likely represents a pseudogene.

Mutations in the structural genes for eukaryotic initiation factors 2 alpha and 2 beta of Saccharomyces cerevisiae disrupt translational control of GCN4 mRNA

The SUI2 and SUI3 genes of Saccharomyces cerevisiae encode the alpha and beta subunits, respectively, of translation initiation factor eIF-2 (eukaryotic initiation factor 2). Previously isolated mutations in these genes restore expression from his4 mutant alleles lacking an ATG initiation codon. The SUI mutations also lead to increased levels of HIS4 mRNA. We show that the latter phenotype exists because the SUI mutations elevate expression of GCN4, an activator of HIS4 transcription. Increased GCN4 expression in the SUI mutants occurs independently of the GCN2 and GCN3 gene products that are normally required to stimulate translation of GCN4 mRNA under conditions of amino acid starvation. Derepression of GCN4 expression in the SUI mutants requires the multiple AUG codons in the leader of the GCN4 transcript that normally mediate its translational control by amino acid availability. In these respects, the SUI mutations resemble mutations in GCD genes whose products function as translational repressors of GCN4. Thus, in addition to its general role in AUG start codon selection, eIF-2 appears to be an important factor in GCN4 translational control. We also show that deletion of GCN3 in sui2-1 strains is lethal, suggesting that GCN3 contributes to eIF-2 alpha function in addition to its role as a translational activator of GCN4.

Control of the Saccharomyces cerevisiae regulatory gene PET494: transcriptional repression by glucose and translational induction by oxygen

The product of the Saccharomyces cerevisiae nuclear gene PET494 is required to promote the translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII). The level of cytochrome c oxidase activity is affected by several different environmental conditions, which also influence coxIII expression. We have studied the regulation of PET494 to test whether the level of its expression might modulate coxIII translation in response to these conditions. A pet494::lacZ fusion was constructed and used to monitor PET494 expression. PET494 was regulated by oxygen availability: expression in a respiration-competent diploid strain grown anaerobically was one-fifth the level of expression in aerobically grown cells. However, since PET494 mRNA levels did not vary in response to oxygen deprivation, regulation by oxygen appears to occur at the translational level. This oxygen regulation was not mediated by heme, and PET494 expression was independent of the heme activator protein HAP2. The regulation of PET494 expression by carbon source was also examined. In cells grown on glucose-containing medium, PET494 was expressed at levels four- to sixfold lower than in cells grown on ethanol and glycerol. However, addition of ethanol to glucose-containing medium induced PET494 expression approximately twofold. PET494 transcript levels varied over a fourfold range in response to different carbon sources. The effects on PET494 expression of mutations in the SNF1, SNF2, SSN6, and HXK2 genes were also determined and found to be minimal.

The first and fourth upstream open reading frames in GCN4 mRNA have similar initiation efficiencies but respond differently in translational control to change in length and sequence

The third and fourth AUG codons in GCN4 mRNA efficiently repress translation of the GCN4-coding sequences under normal growth conditions. The first AUG codon is approximately 30-fold less inhibitory and is required under amino acid starvation conditions to override the repressing effects of AUG codons 3 and 4. lacZ fusions constructed to functional, elongated versions of the first and fourth upstream open reading frames (URFs) were used to show that AUG codons 1 and 4 function similarly as efficient translational start sites in vivo, raising the possibility that steps following initiation distinguish the regulatory properties of URFs 1 and 4. In accord with this idea, we observed different consequences of changing the length and termination site of URF1 versus changing those of URFs 3 and 4. The latter were lengthened considerably, with little or no effect on regulation. In fact, the function of URFs 3 and 4 was partially reconstituted with a completely heterologous URF. By contrast, certain mutations that lengthen URF1 impaired its positive regulatory function nearly as much as removing its AUG codon did. The same mutations also made URF1 a much more inhibitory element when it was present alone in the mRNA leader. These results strongly suggest that URFs 1 and 4 both function in regulation as translated coding sequences. To account for the phenotypes of the URF1 mutations, we suggest the most ribosomes normally translate URF1 and that the mutations reduce the number of ribosomes that are able to complete URF1 translation and resume scanning downstream. This effect would impair URF1 positive regulatory function if ribosomes must first translate URF1 in order to overcome the strong translational block at the 3'-proximal URFs. Because URF1-lacZ fusions were translated at the same rate under repressing and derepressing conditions, it appears that modulating initiation at URF1 is not the means that is used to restrict the regulatory consequences of URF1 translation to starvation conditions.

The positive regulatory function of the 5'-proximal open reading frames in GCN4 mRNA can be mimicked by heterologous, short coding sequences

Translational control of GCN4 expression in the yeast Saccharomyces cerevisiae is mediated by multiple AUG codons present in the leader of GCN4 mRNA, each of which initiates a short open reading frame of only two or three codons. Upstream AUG codons 3 and 4 are required to repress GCN4 expression in normal growth conditions; AUG codons 1 and 2 are needed to overcome this repression in amino acid starvation conditions. We show that the regulatory function of AUG codons 1 and 2 can be qualitatively mimicked by the AUG codons of two heterologous upstream open reading frames (URFs) containing the initiation regions of the yeast genes PGK and TRP1. These AUG codons inhibit GCN4 expression when present singly in the mRNA leader; however, they stimulate GCN4 expression in derepressing conditions when inserted upstream from AUG codons 3 and 4. This finding supports the idea that AUG codons 1 and 2 function in the control mechanism as translation initiation sites and further suggests that suppression of the inhibitory effects of AUG codons 3 and 4 is a general consequence of the translation of URF 1 and 2 sequences upstream. Several observations suggest that AUG codons 3 and 4 are efficient initiation sites; however, these sequences do not act as positive regulatory elements when placed upstream from URF 1. This result suggests that efficient translation is only one of the important properties of the 5' proximal URFs in GCN4 mRNA. We propose that a second property is the ability to permit reinitiation following termination of translation and that URF 1 is optimized for this regulatory function.

Control of the Saccharomyces cerevisiae regulatory gene PET494: transcriptional repression by glucose and translational induction by oxygen

The product of the Saccharomyces cerevisiae nuclear gene PET494 is required to promote the translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII). The level of cytochrome c oxidase activity is affected by several different environmental conditions, which also influence coxIII expression. We have studied the regulation of PET494 to test whether the level of its expression might modulate coxIII translation in response to these conditions. A pet494::lacZ fusion was constructed and used to monitor PET494 expression. PET494 was regulated by oxygen availability: expression in a respiration-competent diploid strain grown anaerobically was one-fifth the level of expression in aerobically grown cells. However, since PET494 mRNA levels did not vary in response to oxygen deprivation, regulation by oxygen appears to occur at the translational level. This oxygen regulation was not mediated by heme, and PET494 expression was independent of the heme activator protein HAP2. The regulation of PET494 expression by carbon source was also examined. In cells grown on glucose-containing medium, PET494 was expressed at levels four- to sixfold lower than in cells grown on ethanol and glycerol. However, addition of ethanol to glucose-containing medium induced PET494 expression approximately twofold. PET494 transcript levels varied over a fourfold range in response to different carbon sources. The effects on PET494 expression of mutations in the SNF1, SNF2, SSN6, and HXK2 genes were also determined and found to be minimal.

sry h-1, a new Drosophila melanogaster multifingered protein gene showing maternal and zygotic expression

Low-stringency hybridization of the Drosophila serendipity (sry) finger-coding sequences revealed copies of homologous DNA sequences in the genomes of members of the family Drosophilidae and higher vertebrates. sry h-1, a new Drosophila finger protein-coding gene isolated on the basis of this homology, encodes a 3.2-kilobase (kb) mRNA accumulating in eggs and abundant in early embryos. The predicted sry h-1 protein product, starting at an internal initiation site of translation, is a 868-amino-acid basic polypeptide containing eight TFIIIA-like fingers encoded by three separate exons. Links separating individual fingers in the sry h-1 protein are variable in length and sequence, in contrast with the invariant H/C link found in most multi-fingered proteins. The similarity of the developmental pattern of transcription of sry h-1 with that of several other Drosophila finger protein genes suggests the existence of a complex set of such genes encoding an information which is, at least partly, maternally provided to the embryo and required for activation of gene transcription in early embryos or maintenance of gene activity during subsequent development.

The first and fourth upstream open reading frames in GCN4 mRNA have similar initiation efficiencies but respond differently in translational control to change in length and sequence

The third and fourth AUG codons in GCN4 mRNA efficiently repress translation of the GCN4-coding sequences under normal growth conditions. The first AUG codon is approximately 30-fold less inhibitory and is required under amino acid starvation conditions to override the repressing effects of AUG codons 3 and 4. lacZ fusions constructed to functional, elongated versions of the first and fourth upstream open reading frames (URFs) were used to show that AUG codons 1 and 4 function similarly as efficient translational start sites in vivo, raising the possibility that steps following initiation distinguish the regulatory properties of URFs 1 and 4. In accord with this idea, we observed different consequences of changing the length and termination site of URF1 versus changing those of URFs 3 and 4. The latter were lengthened considerably, with little or no effect on regulation. In fact, the function of URFs 3 and 4 was partially reconstituted with a completely heterologous URF. By contrast, certain mutations that lengthen URF1 impaired its positive regulatory function nearly as much as removing its AUG codon did. The same mutations also made URF1 a much more inhibitory element when it was present alone in the mRNA leader. These results strongly suggest that URFs 1 and 4 both function in regulation as translated coding sequences. To account for the phenotypes of the URF1 mutations, we suggest the most ribosomes normally translate URF1 and that the mutations reduce the number of ribosomes that are able to complete URF1 translation and resume scanning downstream. This effect would impair URF1 positive regulatory function if ribosomes must first translate URF1 in order to overcome the strong translational block at the 3'-proximal URFs. Because URF1-lacZ fusions were translated at the same rate under repressing and derepressing conditions, it appears that modulating initiation at URF1 is not the means that is used to restrict the regulatory consequences of URF1 translation to starvation conditions.

Molecular characterization of GCD1, a yeast gene required for general control of amino acid biosynthesis and cell-cycle initiation

The GCD1 gene product of Saccharomyces cerevisiae has been implicated in the coordination of the cell cycle with the general control of amino acid biosynthesis (M. Wolfner et al., J. Mol. Biol. 96:273-290, 1975). Strains containing the gcd1-1 allele constitutively express the amino acid biosynthetic genes at the induced levels normally found only during conditions of amino acid starvation. In addition, gcd1-1 strains do not grow at high temperatures because under these conditions they are unable to proceed beyond the START step of the cell division cycle. We have cloned and sequenced the GCD1 gene and examined various aspects of cellular metabolism in order to elucidate its role(s) in regulating gene expression and the cell cycle. GCD1 encodes a 1.7 kb RNA whose expression is not regulated as a function of amino acid starvation. Overexpression of this RNA does not affect the regulation of amino acid biosynthetic genes or cell growth. GCD1 is an essential gene because cells containing a gcd1-HIS3 disruption are unable to grow. The essential function of GCD1 may be involved in protein synthesis because a gcd1-1 strain incorporates low levels of 35S-methionine into protein when cells are shifted to the restrictive temperature. GCD1 encodes a protein of 511 amino acids whose predicted sequence does not exhibit significant homology to any other known proteins and appears too large to be a ribosomal protein. We suggest that GCD1 encodes a component of the normal protein synthesis machinery that is involved in the translational regulation of GCN4, a protein that coordinately activates the transcription of amino acid biosynthetic genes. GCD1 may also be part of a sensing mechanism in which cells monitor the protein synthesis capacity prior to initiating a new cell division cycle.

sry h-1, a new Drosophila melanogaster multifingered protein gene showing maternal and zygotic expression

Low-stringency hybridization of the Drosophila serendipity (sry) finger-coding sequences revealed copies of homologous DNA sequences in the genomes of members of the family Drosophilidae and higher vertebrates. sry h-1, a new Drosophila finger protein-coding gene isolated on the basis of this homology, encodes a 3.2-kilobase (kb) mRNA accumulating in eggs and abundant in early embryos. The predicted sry h-1 protein product, starting at an internal initiation site of translation, is a 868-amino-acid basic polypeptide containing eight TFIIIA-like fingers encoded by three separate exons. Links separating individual fingers in the sry h-1 protein are variable in length and sequence, in contrast with the invariant H/C link found in most multi-fingered proteins. The similarity of the developmental pattern of transcription of sry h-1 with that of several other Drosophila finger protein genes suggests the existence of a complex set of such genes encoding an information which is, at least partly, maternally provided to the embryo and required for activation of gene transcription in early embryos or maintenance of gene activity during subsequent development.

Scanning independent ribosomal initiation of the Sendai virus X protein

Both peptide antisera and monoclonal antibodies detect a 10-kd protein (X) in Sendai virus infected cells, which represents approximately the last 95 residues of the 568-amino-acid-long P protein. The X protein does not appear to be derived from a precursor P in vivo, and in in vitro X can be made under conditions in which P synthesis has been arrested by hybridized DNA fragments or specific cleavage of the mRNA. X initiation is nevertheless cap dependent. Ribosomes which initiate X appear to pass directly from the cap to the initiation codon.

The positive regulatory function of the 5'-proximal open reading frames in GCN4 mRNA can be mimicked by heterologous, short coding sequences

Translational control of GCN4 expression in the yeast Saccharomyces cerevisiae is mediated by multiple AUG codons present in the leader of GCN4 mRNA, each of which initiates a short open reading frame of only two or three codons. Upstream AUG codons 3 and 4 are required to repress GCN4 expression in normal growth conditions; AUG codons 1 and 2 are needed to overcome this repression in amino acid starvation conditions. We show that the regulatory function of AUG codons 1 and 2 can be qualitatively mimicked by the AUG codons of two heterologous upstream open reading frames (URFs) containing the initiation regions of the yeast genes PGK and TRP1. These AUG codons inhibit GCN4 expression when present singly in the mRNA leader; however, they stimulate GCN4 expression in derepressing conditions when inserted upstream from AUG codons 3 and 4. This finding supports the idea that AUG codons 1 and 2 function in the control mechanism as translation initiation sites and further suggests that suppression of the inhibitory effects of AUG codons 3 and 4 is a general consequence of the translation of URF 1 and 2 sequences upstream. Several observations suggest that AUG codons 3 and 4 are efficient initiation sites; however, these sequences do not act as positive regulatory elements when placed upstream from URF 1. This result suggests that efficient translation is only one of the important properties of the 5' proximal URFs in GCN4 mRNA. We propose that a second property is the ability to permit reinitiation following termination of translation and that URF 1 is optimized for this regulatory function.

Transcriptional-translational regulatory circuit in Saccharomyces cerevisiae which involves the GCN4 transcriptional activator and the GCN2 protein kinase

GCN4 protein mediates the transcriptional activation of amino acid biosynthetic genes in Saccharomyces cerevisiae by specifically binding to DNA sequences in their 5'-regulatory regions. GCN4 expression is regulated at the level of translation, with translational derepression occurring under conditions of amino acid starvation. The product of the GCN2 gene is essential for translational derepression of GCN4. Sequence analysis of the GCN2 gene reveals that the GCN2 protein has a domain highly homologous to the catalytic domain of all known protein kinases. Furthermore, gcn2 strains are deficient in a protein kinase activity corresponding to a protein with the calculated molecular weight deduced from the GCN2 open reading frame. Therefore it is likely that GCN2 encodes a protein kinase, which may be directly involved in translational regulation of the GCN4 mRNA. Transcription of the GCN2 gene is increased when cells are cultured in amino acid starvation medium. This transcriptional activation is mediated by the GCN4 protein, which binds to the promoter region of the GCN2 gene. Thus, this system is modulated by a transcriptional-translational regulatory circuit, which is activated by amino acid starvation. Activation is not the result of a simple quantitative increase of either one of the identified components of the circuit.

Internal initiation of translation of eukaryotic mRNA directed by a sequence derived from poliovirus RNA

Poliovirus RNA is naturally uncapped, therefore its translation must proceed via a cap-independent mechanism. Translation initiation on poliovirus RNA occurs by binding of ribosomes to an internal sequence within the 5' noncoding region. This novel mechanism of initiation may explain the disparate translation of several other eukaryotic messenger RNAs.

Transcriptional-translational regulatory circuit in Saccharomyces cerevisiae which involves the GCN4 transcriptional activator and the GCN2 protein kinase

GCN4 protein mediates the transcriptional activation of amino acid biosynthetic genes in Saccharomyces cerevisiae by specifically binding to DNA sequences in their 5'-regulatory regions. GCN4 expression is regulated at the level of translation, with translational derepression occurring under conditions of amino acid starvation. The product of the GCN2 gene is essential for translational derepression of GCN4. Sequence analysis of the GCN2 gene reveals that the GCN2 protein has a domain highly homologous to the catalytic domain of all known protein kinases. Furthermore, gcn2 strains are deficient in a protein kinase activity corresponding to a protein with the calculated molecular weight deduced from the GCN2 open reading frame. Therefore it is likely that GCN2 encodes a protein kinase, which may be directly involved in translational regulation of the GCN4 mRNA. Transcription of the GCN2 gene is increased when cells are cultured in amino acid starvation medium. This transcriptional activation is mediated by the GCN4 protein, which binds to the promoter region of the GCN2 gene. Thus, this system is modulated by a transcriptional-translational regulatory circuit, which is activated by amino acid starvation. Activation is not the result of a simple quantitative increase of either one of the identified components of the circuit.

Translational activation of the lck proto-oncogene

The lck proto-oncogene, a member of the src gene family, encodes a lymphocyte-specific protein tyrosine kinase (p56lck) that is implicated in the pathogenesis of lymphoid neoplasia. We report here that 5' lck sequence elements, containing AUG codons, significantly reduce the in vivo efficiency of p56lck translation from the normal messenger RNA. This result provides a quantitative explanation for the overexpression of p56lck in two retrovirally-induced murine lymphomas that express abnormal lck transcripts from which the physiological 5' translational control region has been deleted and non AUG-containing viral sequences substituted. In contrast to other mammalian genes, most proto-oncogenes contain AUG codons 5' to the authentic initiation codon, suggesting that cells can regulate translational start sites in these mRNAs. Abrogation of translational control may be a general mechanism of proto-oncogene activation in malignancy.

The structure and expression of a novel gene activated in early mouse embryogenesis

We report the cloning and sequence determination of the mouse H19 gene. This gene is under the genetic control of two trans-acting loci in the mouse, termed raf and Rif. These loci determine the adult basal and inducible levels, respectively, of H19 mRNA, as well as the mRNA for alpha-fetoprotein. By elucidating the sequence and structure of the H19 gene we show that it is unrelated to the alpha-fetoprotein gene, and therefore must have acquired its regulation by raf and Rif independently. The sequence also indicates that the H19 gene has a very unusual structure. It is composed of five exons, 1307, 135, 119, 127 and 560 bp in size, along with four very small introns whose combined lengths are 270 bases. The largest open reading frame of the gene, sufficient to encode a protein of approximately 14 kd, is contained entirely within the first large exon, 680 bases downstream of the cap site of the mRNA. Preceding the translation initiation codon are four ATG codons, each of which is followed shortly thereafter by translation terminator codons. The rest of the gene, which encompasses all five exons, is presumed to be untranslated. That the long 5' untranslated region may be used to regulate the translation of the mRNA is suggested from in vitro translation studies. Experiments which utilized tissue culture cell lines of the mesodermal lineage suggest that the gene is activated very early during muscle cell differentiation.

A segment of GCN4 mRNA containing the upstream AUG codons confers translational control upon a heterologous yeast transcript

GCN4 encodes a transcriptional activator in Saccharomyces cerevisiae that is regulated at the translational level. We show that an approximately 240-nucleotide segment from the GCN4 mRNA leader containing four AUG codons is sufficient to confer translational control typical of GCN4 upon a GAL1-lacZ fusion transcript. Regulation of the hybrid transcript is dependent upon multiple positive (GCN) and negative (GCD) trans-acting factors shown to regulate GCN4 expression post-transcriptionally. This result limits the target sequences for these factors to a small internal segment of the GCN4 mRNA leader. The elimination of AUG codons within this segment substantially reduces the usual derepressing effect of mutations in five GCD genes upon GCN4-lacZ expression. This supports the idea that the products of these negative regulatory genes act by modulating the effects of the upstream AUG codons on translation of GCN4 mRNA.

Optional exon in the 5'-untranslated region of 3-hydroxy-3-methylglutaryl coenzyme A synthase gene: conserved sequence and splicing pattern in humans and hamsters

3-Hydroxy-3-methylglutaryl coenzyme A synthase (hydroxymethylglutaryl-CoA synthase, EC 4.1.3.5) is a negatively regulated enzyme in the synthetic pathway for cholesterol, isopentenyl tRNA, and other isoprenoids. The 5'-untranslated region of the mRNA for Chinese hamster hydroxymethylglutaryl-CoA synthase contains an optional exon of 59 nucleotides located 10 nucleotides upstream of the translation start site. About 50% of the mRNAs contain this exon, and the other 50% lack it owing to differential intron splicing. We show that the two transcripts are found in similar ratios in multiple tissues of the Syrian hamster, including the brain. The relative amounts of the two transcripts in brain and liver are constant from day 0 to day 75 of life. A similar alternative splicing pattern for hydroxymethylglutaryl-CoA synthase was observed in three human tissues: cultured fibroblasts, fetal adrenal gland, and fetal liver. A cDNA for human synthase had 90% homology to the hamster sequence in the region corresponding to the optional exon. This sequence contains a 20 out of 26 nucleotide match with the sequence immediately upstream of the initiator AUG codon in the mRNA for hamster hydroxymethylglutaryl-CoA reductase, the enzyme that follows the synthase in the isoprenoid biosynthetic pathway. These findings raise the possibility that the optional exon plays an important, conserved functional role in humans and hamsters.