In vitro translation products specified by the transforming region of adenovirus type 2

Two-step transformation of rat 3Y1 cells by the adenovirus E1A and E1B genes

The transformation of rodent cells by the adenovirus E1A and E1B genes was very efficient when these genes were physically linked. When they were cleaved, the transformation became very inefficient. To clarify this difference, the chimeric E1B genes in which either the adenovirus enhancer or the human beta-actin promoter was linked to the 5' side of the E1B gene were introduced into rat 3Y1 cells. The saturation density of these cell lines (eB or APrB) was similar to that of parental 3Y1 cells. When eB or APrB cell lines were supertransfected with the E1A gene, discrete dense foci were developed after 5-6 weeks, while the supertransfection of 3Y1 derivative cell lines, in which the enhancer-unlinked E1B gene was introduced, did not develop any dense foci. Analysis of the E1A and E1B transcripts in these cell lines indicated that the E1B gene is efficiently expressed in the presence of the E1A gene products if the enhancer is linked to the E1B gene and that an increased level of E1B proteins is required for an efficient expression of the E1A gene. These results indicated that E1A and E1B genes in separate pieces of DNA are capable of cooperatively transforming 3Y1 cells if appropriate cis-acting elements are attached and high-level expressions are achieved.

Nuclear transport of proteins translated in vitro from SP6 plasmid-generated mRNAs

A sensitive and versatile assay is described for the nuclear transport of 35S-labeled proteins obtained by the in vitro translation of SP6 plasmid-generated mRNAs. A specific nuclear accumulation of greater than 20-fold is observed for the transformation-related nuclear proteins, p53 and E1b, and the nuclear enzyme, thymidine kinase, whereas transport of the nonnuclear proteins, dihydrofolate reductase and simian virus 40 small t antigen, is negligible within 30 min.

Nuclear transport of proteins translated in vitro from SP6 plasmid-generated mRNAs

A sensitive and versatile assay is described for the nuclear transport of 35S-labeled proteins obtained by the in vitro translation of SP6 plasmid-generated mRNAs. A specific nuclear accumulation of greater than 20-fold is observed for the transformation-related nuclear proteins, p53 and E1b, and the nuclear enzyme, thymidine kinase, whereas transport of the nonnuclear proteins, dihydrofolate reductase and simian virus 40 small t antigen, is negligible within 30 min.

Organization of the transcriptional control region of the E1b gene of adenovirus type 5

Genetic analysis of the transcriptional control sequences of the E1b gene of adenovirus type 5 identified two regions that stimulated specific transcription by whole cell extracts from uninfected cells. The first region, located within 50 nucleotides (position -50) 5' to the transcription initiation (cap) site, contains a G+C-rich consensus-binding site (GC box) for the transcription factor Sp1 and a TATA box. Unambiguous stimulatory activity of the second region, between positions -358 and -127, was observed only in the absence of the GC box. DNase I protection experiments (footprinting) with crude nuclear extracts from uninfected cells revealed multiple DNA-protein interactions at the control region. Proximal to the initiation site, both the GC box and the cap site were protected; however, protection of the TATA box was not observed. In the distal region, four protein-binding sites, designated I through IV, were located between positions -250 and -120. Three of the four mapped in protein-coding sequences of the adjacent E1a gene. Sites I and II were 5' to position -218 whereas sites III and IV were 3' to position -218. This finding was consistent with results of the transcriptional analysis indicating that subsets of the distal region were sufficient for stimulation of transcription in vitro in the absence of the GC box. Within the boundaries of site I, a 10-base-pair protected sequence was similar to one located 5' to the adenovirus E1a, E2a, E3, E4, E2 late, and polypeptide IX transcription initiation sites. Sequences within the boundaries of the other three sites were similar to those within other viral and cellular enhancers.

Detailed kinetics of adenovirus type-5 steady-state transcripts during early infection

The appearance and steady-state accumulation of specific viral RNAs during the early phase of adenovirus type 5 (Ad5) infection was examined. HeLa cells were synchronously infected and harvested at 30 min intervals throughout the first 12 h of infection. Total cytoplasmic RNA was extracted from infected cells and analyzed by hybridization-selection and translation to identify the viral mRNAs from each early region on the basis of the protein products they encode. The same RNA samples were used for S-1 nuclease and Northern blot analyses to quantitatively compare the levels of individual viral RNAs that accumulate within each early transcription region (E1A, E1B, L1, E2A, E3 and E4). The salient features of this analysis show that RNA accumulation occurs first from E1A followed by E2A, E3 and E4, E1B and lastly, L1. Although the profile of RNA accumulation was unique for each early region, overlapping RNAs within E1A, E3, and E4, respectively, remained generally parallel to one another throughout early infection, in contrast to RNAs from E1B and L1, respectively. Since both the appearance and quantitative accumulation of specific early viral mRNAs were examined at many time points, a number of subtleties associated with the complex dynamics of early Ad5 gene expression were revealed. In particular, the L1 region was shown to transcribe from the major late promoter two early RNAs of 3.81 Kb and 3.5 Kb, either or both of which encode the 52,55 kDa proteins; the auxiliary i leader sequence was found on the 3.81 Kb RNA but not on the 3.5 Kb RNA.

Two regions of the adenovirus early region 1A proteins are required for transformation

Regions of the adenovirus type 5 early region 1A (E1A) proteins that are required for transformation were defined by using a series of deletion mutants. Deletion mutations collectively spanning the entire protein-coding region of E1A were constructed and assayed for their ability to cooperate with an activated ras oncogene to induce transformation in primary baby rat kidney cells. Two regions of E1A (amino acids 1 to 85 and 121 to 127) were found to be essential for transformation. Deletion of all or part of the region from amino acids 121 to 127 resulted in a total loss of transforming ability. An adjacent stretch of amino acids (residues 128 to 139), largely consisting of acidic residues, was found to be dispensable for transformation but appeared to influence the efficiency of transformation. Amino acids 1 to 85 made up a second region of the E1A protein that was essential for transformation. Deletion of all or part of this region resulted in a loss of the transforming activity. Even a mutation resulting in a single amino acid change at position 2 of the polypeptide chain was sufficient to eliminate transformation. Deletion of amino acids 86 to 120 or 128 to 289 did not eliminate transformation, although some mutations in these regions had lowered efficiencies of transformation. Foci induced by transformation-competent mutants could be expanded into cell lines that retained their transformed morphology and constitutively expressed the mutant E1A proteins.

Differential splicing yields novel adenovirus 5 E1A mRNAs that encode 30 kd and 35 kd proteins

In addition to the protein products of the adenovirus E1A 13S and 12S mRNAs, monoclonal antibodies specific for the E1A proteins immunoprecipitate polypeptides with relative mol. wt of 30,000 (30 kd) and 35,000 (35 kd) from extracts of infected cells. The 30 kd and 35 kd proteins are encoded by novel mRNAs referred to as the 10S and 11S mRNAs, respectively. These two mRNAs arise from differential splicing of the E1A precursor RNA. For the 10S mRNA, the precursor is spliced twice, once removing the region between nucleotides 637 and 854 and once between 974 and 1229. The splice between nucleotides 974 and 1229 is identical to the one used for the processing of the 12S mRNA. Synthesis of the 11S mRNA also utilizes two splicing events. One of these is identical to the 637/854 splice of the 10S mRNA, and the other removes the region between nucleotides 1112 and 1229, a splice junction also found in the 13S mRNA. All four mRNAs used the same reading frame and, therefore, code for related proteins. The products of the 10S and 11S mRNAs are identical to the products of the 12S and 13S mRNAs, respectively, except for an internal stretch of 27 amino acids removed by the 637/854 splice. Within this segment is a group of amino acid residues that is highly conserved between different adenovirus serotypes. Mutant adenoviruses in which the wild-type E1A sequences have been replaced with cDNA copies of the 10S or 11S mRNAs are defective for growth on HeLa cells suggesting that this region is important for viral growth.(ABSTRACT TRUNCATED AT 250 WORDS)

tsJT60, a cell cycle G0-ts mutant, becomes lethal at non-permissive temperature by transformation with adenovirus 5 when the expression of E1B gene is lacking

tsJT60, a temperature-sensitive (ts) mutant cell line of Fischer rat, is viable at both permissive (34 degrees C) and non-permissive (39.5 degrees C) temperatures. The cells grow normally in exponential growth phase at both temperatures, but when stimulated with fetal bovine serum (FBS) from G0 phase they re-enter S phase at 34 degrees C but not at 39.5 degrees. When tsJT60 cells were transformed with adenovirus (Ad) 5 wild type, they grew well at both temperatures, expressed E1A and E1B genes, and formed colonies in soft agar. When tsJT60 cells were transformed with Ad5 dl313, that lacks E1B gene, the transformed cells grew well at 34 degrees C but failed to form colony in soft agar. They died very soon at 39.5 degrees C. 3Y1 cells (a parental line of tsJT60) transformed with dl313 grew well at both temperatures, although neither expressed E1B gene nor formed colonies in soft agar. The phenotype of being lethal at 39.5 degrees C of dl313-transformed tsJT60 cells was complemented by cell fusion with 3Y1BUr cells (5-BrdU-resistant 3Y1), but not with tsJT60TGr cells (6-thioguanine resistant tsJT60). These results indicate that the lethal phenotype is related to the ts mutation of tsJT60 cells and also to the deletion of E1B gene of Ad5.

Association of adenovirus early-region 1A proteins with cellular polypeptides

Extracts from adenovirus-transformed human 293 cells were immunoprecipitated with monoclonal antibodies specific for the early-region 1A (E1A) proteins. In addition to the E1A polypeptides, these antibodies precipitated a series of proteins with relative molecular weights of 28,000, 40,000, 50,000, 60,000, 80,000, 90,000, 110,000, 130,000, and 300,000. The two most abundant of these polypeptides are the 110,000-molecular-weight protein (110K protein) and 300K protein. Three experimental approaches have suggested that the 110K and 300K polypeptides are precipitated because they form stable complexes with the E1A proteins. The 110K and 300K polypeptides do not share epitopes with the E1A proteins, they copurify with a subset of the E1A proteins, and they bind to the E1A proteins following mixing in vitro. The 110K and 300K polypeptides are not adenoviral proteins, but are encoded by cellular DNA. Both the 12S and the 13S E1A proteins bind to the 110K and 300K species, and these complexes are found in adenovirus-transformed and -infected cells.

Sequence analysis in the E1 region of adenovirus type 4 DNA

Adenovirus type 4 (Ad4) is the sole member of adenovirus group E based on overall DNA sequence homology, restriction endonuclease cleavage patterns, and the size of capsid proteins. We cloned the BamHI-F fragment from the left end of Ad4 in pUC13-1 between the SalI and BamHI sites in order to carry out the structural analysis of the E1A region of Ad4. The complete sequence of the BamHI-F fragment (2042 bp) has been determined. From the DNA sequence, the splice sites for the putative 12 S and 13 S mRNAs, encoded by the E1A region of Ad4 were deduced. If protein synthesis initiates at the first available AUG triplet (position 575), these 12 S and 13 S mRNAs would code for polypeptides containing 226 and 257 amino acids, respectively. Comparison of Ad4- and Ad7-13 S mRNA-coded polypeptides indicates that there is 57% homology, whereas the homology is only 38% with Ad12 and 31% with Ad2-13 S mRNA-coded polypeptides. The structural analysis in the E1 region of Ad4 also includes the coding region for the E1B 19-kDa protein. Ad4 and Ad7 shows 65% homology in the coding regions for E1B 19-kDa protein. Comparison of the DNA sequence of Ad4 with those of Ad2, Ad7, and Ad12 by using a dot matrix computer program and by Southern hybridization revealed that Ad4 bears a stronger homology with Ad7 than with Ad2 and Ad12 in this region. Hydropathy plots and alignments of the putative polypeptides coded by this region in Ad4 with those from the corresponding regions of different serotypes to reveal the highly conserved domains also support the above conclusion.

Partition of E1A proteins between soluble and structural fractions of adenovirus-infected and -transformed cells

The partition of E1A proteins between soluble and structural framework fractions of human cells infected or transformed by subgroup C adenoviruses was investigated by using gentle cell fractionation conditions. A polyclonal antibody raised against a trpE-E1A fusion protein (K.R. Spindler, D.S.E. Rosser, and A. J. Berk, J. Virol. 132-141, 1984) synthesized in Escherichia coli was used to measure the steady-state levels of E1A proteins recovered in the various fractions by immunoblotting. The relative concentration of E1A proteins recovered in the soluble fraction of adenovirus type 2-infected cells was at least fivefold greater than the relative concentration in the corresponding fraction of transformed 293 cells. The observed distribution of E1A proteins was not altered by the sulfhydryl-blocking reagent N-ethylmaleimide. E1A proteins were recovered in nuclear matrix, chromatin, and cytoskeleton fractions after further fractionation of the structural framework fraction. However, the E1A protein species that could be identified by one-dimensional gel electrophoresis were not uniformly distributed among the subcellular fractions examined. The results obtained when fractionation was performed in the presence of the oxidation catalysts Cu2+ or (ortho-phenanthroline)2 Cu2+ indicate that E1A proteins can be efficiently cross-linked, via disulfide bonds, to the structural framework of both adenovirus-infected and adenovirus-transformed cells.

Association of adenovirus early-region 1A proteins with cellular polypeptides

Extracts from adenovirus-transformed human 293 cells were immunoprecipitated with monoclonal antibodies specific for the early-region 1A (E1A) proteins. In addition to the E1A polypeptides, these antibodies precipitated a series of proteins with relative molecular weights of 28,000, 40,000, 50,000, 60,000, 80,000, 90,000, 110,000, 130,000, and 300,000. The two most abundant of these polypeptides are the 110,000-molecular-weight protein (110K protein) and 300K protein. Three experimental approaches have suggested that the 110K and 300K polypeptides are precipitated because they form stable complexes with the E1A proteins. The 110K and 300K polypeptides do not share epitopes with the E1A proteins, they copurify with a subset of the E1A proteins, and they bind to the E1A proteins following mixing in vitro. The 110K and 300K polypeptides are not adenoviral proteins, but are encoded by cellular DNA. Both the 12S and the 13S E1A proteins bind to the 110K and 300K species, and these complexes are found in adenovirus-transformed and -infected cells.

Posttranslational modification at the N terminus of the human adenovirus type 12 E1A 235R tumor antigen

The adenovirus E1A transforming region, which encodes immortalization, partial cell transformation, and gene activation functions, expresses two early mRNAs, 13S and 12S. Multiple-T antigen species with different electrophoretic mobilities are formed from each mRNA, presumably by unknown posttranslational modifications. The adenovirus type 12 (Ad12) 13S and 12S mRNAs encode E1A T antigens of 266 and 235 amino acid residues (266R and 235R), respectively. To study possible posttranslational processing at the N and C termini and to distinguish between the Ad12 266R and 235R T antigens, we prepared antibodies targeted to synthetic peptides encoded at the common C (peptide 204) and N (peptide 202) termini of the 266R and 235R T antigens and at the unique internal domain of the 266R T antigen (peptide 206). The specificity of each anti-peptide antibody was confirmed by immunoprecipitation of the 266R and 235R T antigens produced in Escherichia coli. Immunoprecipitation analysis of the E1A T antigens synthesized in Ad12-infected KB cells revealed the following. Antibody to the common C terminus recognized three T antigens with apparent Mrs of 43,000, 42,000, and 39,000 (43K, 42K, and 39K). All three forms were phosphorylated and were present in both the nucleus and the cytoplasm. The 43K and 42K T antigens were rapidly synthesized during a 10-min pulse with [35S]methionine in Ad12-infected cells. The 43K T antigen had a half-life of 20 min, the 42K T antigen had a longer half-life of about 40 min, and the 39K T antigen became the predominant E1A T antigen. Antibodies to the unique region immunoprecipitated the 43K T antigen but not the 42K and 39K T antigens. Antibody to the N terminus immunoprecipitated the 43K and 42K T antigens but not the 39K T antigen, suggesting that the 39K T antigen possessed a modified N terminus. Partial N-terminal amino acid sequence analysis showed that the 43K and 42K T antigens contain methionine at residues 1 and 5, as predicted from the DNA sequence, whereas no methionine was released from the 39K T antigen during the first six cycles of Edman degradation. We propose that the short-lived 43K T antigen is the primary product of the 13S mRNA, the 266R T antigen; the somewhat more stable 42K T antigen is the primary product of the 12S mRNA, the 235R T antigen.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the adenovirus early region 1B tumor antigens in transformation and lytic infection

We have investigated the contribution of each of the two adenovirus type 5 (Ad5) major early region 1b (E1b) proteins in cell transformation and in lytic infection. An Ad5 E1 plasmid, in which the reading frame for the 19-kDa E1b protein was abolished by a stop codon close to the initiation codon, transformed primary baby rat kidney (BRK) cells with an efficiency of about half of that of a wild type Ad5 E1 plasmid, whereas a plasmid with a mutation in the gene for the 58-kDa E1b protein transformed the same primary cells with only one-third of the wild type efficiency. Plasmids containing region E1a only or a plasmid carrying mutations in the genes for major E1b proteins all transformed primary cells with an efficiency of approximately 5% of wild type. To test the effect of the E1b mutations in virion-mediated cell transformation, the mutant E1b regions were introduced into intact viral genomes by overlap recombination and were subsequently used in a transformation assay on BRK cells. The 19 and 58-kDa mutant viruses were found to transform BRK cells with 11 and 25% of the efficiency of wild type virus, respectively. These results suggest that the 19-kDa E1b protein is essential for virus-mediated cell transformation, in agreement with results of others, but not for plasmid-mediated cell transformation. In lytic infection, the 19-kDa mutant virus was some 30-fold reduced in yield on HeLa cells, whereas the 58-kDa mutant virus was 3000-fold reduced in its ability to grow on HeLa cells at low multiplicity of infection, but showed a marked multiplicity-dependent leakiness. The 58-kDa mutant virus was not defective when its growth was assayed on human embryonic kidney (HEK) cells. This may indicate that cellular proteins are expressed in HEK cells that are functionally homologous to the 58-kDa E1b protein.

A first exon-encoded domain of E1A sufficient for posttranslational modification, nuclear-localization, and induction of adenovirus E3 promoter expression in Xenopus oocytes

The purified Escherichia coli-expressed human subgroup C adenovirus E1A 13S mRNA product induces expression from the adenovirus type 5 E3 promoter when injected into Xenopus oocytes. In the present communication, the E. coli-expressed E1A 13S and 12S mRNA products are shown to undergo a posttranslational modification in microinjected Xenopus oocytes, which causes a 2- to 4-kDa increase in apparent molecular size, identical to that occurring in HeLa cells expressing the E1A gene. The E. coli-expressed E1A proteins are similarly modified in vitro in a soluble rabbit reticulocyte lysate. The modified form of the E1A proteins preferentially localizes to the oocyte nucleus following cytoplasmic microinjection. The use of various deleted forms of E1A protein synthesized in E. coli shows that a first exon-encoded domain of E1A, residing between amino acid residues 23 and 120, is sufficient for the posttranslational modification and nuclear localization of E1A and also for the trans-activation of the E3 promoter by E1A in Xenopus oocytes. These results suggest that the posttranslational modification of E1A protein may be important for its function.

Monoclonal antibodies specific for adenovirus early region 1A proteins: extensive heterogeneity in early region 1A products

Hybridomas secreting monoclonal antibodies specific for the adenovirus early region 1A (E1A) proteins were prepared from BALB/c mice immunized with a bacterial trpE-E1A fusion protein. This protein is encoded by a hybrid gene that joins a portion of the Escherichia coli trpE gene and a cDNA copy of the E1A 13S mRNA (Spindler et al., J. Virol. 49:132-141, 1984). Eighty-three hybridomas that secrete antibodies which recognize the immunogen were isolated and single cell cloned. Twenty-nine of these antibodies are specific for the E1A portion of the fusion protein. Only 12 of the monoclonal antibodies can efficiently immunoprecipitate E1A polypeptides from detergent lysates of infected cells. E1A polypeptides were analyzed on one-dimensional, sodium dodecyl sulfate-polyacrylamide gels and two-dimensional, isoelectric focusing polyacrylamide gels. The E1A proteins that are specifically immunoprecipitated by the monoclonal antibodies are heterogeneous in size and charge and can be resolved into approximately 60 polypeptide species. This heterogeneity is due not only to synthesis from multiple E1A mRNAs, but also at least in part to post-translational modification. Several of the monoclonal antibodies divide the E1A polypeptides into immunological subclasses based on the ability of the antibodies to bind to the antigen. In particular, two of the monoclonal antibodies bind to the polypeptides synthesized from the 13S E1A mRNA, but not to other E1A proteins.

Regulation of transcription of the adenovirus EII promoter by EIa gene products: absence of sequence specificity

During adenovirus infection, the EII promoter is positively regulated by products of the EIa region. We have studied this regulation by fusing a DNA segment containing the adenovirus EII promoter to a dihydrofolate reductase cDNA segment. Expression of this hybrid gene is stimulated in trans when cell lines containing an integrated copy are either transfected with plasmids carrying the EIa region or infected with adenovirus. This suggests that EIa activity regulates transcription of the EII promoter in the absence of other viral proteins and that this stimulation can occur when the EII promoter is organized in cellular chromatin. Transcription from the EII promoter is initiated at two sites in cell lines lacking EIa activity. Introduction of the EIa region preferentially stimulated transcription from one of these two sites. A sensitive, stable cotransfection assay was used to test for specific EII sequences required for stimulation. EIa activity stimulates all mutant promoters; the most extensive deletion retained only 18 base pairs of sequences upstream of the initiation site. We suggest that regulation of a promoter by the EIa region does not depend on the presence of a set of specific sequences, but instead reflects a characteristic of promoters that have been exogenously introduced into cells. Insertion of the 72-base-pair repeat of simian-virus 40 in cis enhances transcription from the EII promoter. The stimulatory effects of EIa activity and of the simian virus 40 sequence are additive and appear to differ mechanistically.

Effect on transformation of mutations in the early region 1b-encoded 21- and 55-kilodalton proteins of adenovirus 5

It is well established that the adenovirus 5 genes responsible for the initiation and maintenance of the transformed cell reside in the early region 1a and 1b genes, but it remains unclear how the polypeptides encoded in these genes mediate their functions. To probe the function of the early region 1b-encoded 55- and 21-kilodalton (kd) polypeptides during this process, a series of viral mutants was engineered so that they contained deletions or insertions at 5.4, 5.7, 7.9, or 9.6 map units. By means of either an overlap recombination procedure involving H5dl314 (delta 3.7 to 4.6 map units) cleaved with ClaI, or a marker rescue procedure involving H5dl312 (delta 1.2 to 3.8 map units), viral mutants were isolated by their ability to produce plaques on KB cell line 18 cells, which constitutively express only viral early region 1b functions. DNA sequence analysis confirmed that the series of mutants generated differed in their abilities to express the 21- or the 55-kd polypeptides, or both. Upon infection of cloned rat embryo fibroblast cells with viruses containing mutations affecting the 55-kd protein, the transformation frequency decreased as the size of the predicted truncated polypeptide decreased. Although all of the foci generated by the 55-kd protein mutants were indistinguishable from the foci induced by wild-type virus, they displayed an inefficient ability to grow in soft agar, again in relation to the size of the truncated polypeptide. In contrast, if cloned rat embryo fibroblast cells were transfected with viral DNA, the defectiveness in transformation observed after infection with virions was not as dramatic. However, all of the viruses containing 21-kd mutations were transformation defective, regardless of the mode by which the viral nucleic acid was introduced into the cell.

Regulation of transcription of the adenovirus EII promoter by EIa gene products: absence of sequence specificity

During adenovirus infection, the EII promoter is positively regulated by products of the EIa region. We have studied this regulation by fusing a DNA segment containing the adenovirus EII promoter to a dihydrofolate reductase cDNA segment. Expression of this hybrid gene is stimulated in trans when cell lines containing an integrated copy are either transfected with plasmids carrying the EIa region or infected with adenovirus. This suggests that EIa activity regulates transcription of the EII promoter in the absence of other viral proteins and that this stimulation can occur when the EII promoter is organized in cellular chromatin. Transcription from the EII promoter is initiated at two sites in cell lines lacking EIa activity. Introduction of the EIa region preferentially stimulated transcription from one of these two sites. A sensitive, stable cotransfection assay was used to test for specific EII sequences required for stimulation. EIa activity stimulates all mutant promoters; the most extensive deletion retained only 18 base pairs of sequences upstream of the initiation site. We suggest that regulation of a promoter by the EIa region does not depend on the presence of a set of specific sequences, but instead reflects a characteristic of promoters that have been exogenously introduced into cells. Insertion of the 72-base-pair repeat of simian-virus 40 in cis enhances transcription from the EII promoter. The stimulatory effects of EIa activity and of the simian virus 40 sequence are additive and appear to differ mechanistically.

Requirement for either early region 1a or early region 1b adenovirus gene products in the helper effect for adeno-associated virus

Several adenovirus early genes act together to promote growth of the helper-dependent adeno-associated virus (AAV). Data from several laboratories have implicated adenovirus early regions 1a, 1b, 2a, and 4 in the helper effect, as well as the small RNA polymerase III transcript, virus-associated RNA I. Although a subset of these must participate directly in the AAV life cycle, some may play an indirect role by influencing expression of the others. This paper is concerned particularly with the roles of early regions 1a and 1b in the helper effect. We introduced DNA fragments representing the various early regions into AAV-infected or uninfected Vero cells, by the manual microinjection procedure. After labeling the cells with [35S]methionine, we visualized immunoprecipitates of AAV or adenovirus proteins on sodium dodecyl sulfate-polyacrylamide gels. When over 200 copies of each DNA fragment per cell were injected, early regions 2a and 4 were themselves sufficient to provide the helper effect. At 100 copies per cell, however, a third gene became essential, and this could be either early region 1a or 1b. The role of early region 1a is easily explained by its known ability to stimulate transcription of the other early genes. The function of early region 1b is less clear, but it does not simply mimic the action of early region 1a. Instead, there appear to be at least two distinct regulatory pathways which can lead to expression of AAV. To investigate the sequence of regulatory interactions, we microinjected purified adenovirus mRNAs, or combinations of mRNA and DNA, into AAV-infected cells. Our results suggest that adenovirus early products enhance viral gene expression by several mechanisms which can operate independently, but whose effects may be cumulative.

Adenovirus E1a gene product expressed at high levels in Escherichia coli is functional

The human type C adenovirus E1a 13S messenger RNA encodes a gene product, that positively regulates the transcription of viral genes and certain cellular genes and is involved in the transformation of primary mammalian cells. The E1a gene product was expressed at high levels in Escherichia coli. In a Xenopus oocyte microinjection assay, the purified Escherichia coli-produced protein activated the E1a-responsive adenovirus E3 promoter and functioned as efficiently as the E1a gene itself.

Early region 1B of adenovirus 2 encodes two coterminal proteins of 495 and 155 amino acid residues

Partial sequence analysis of tryptic peptides has identified the E1B-495R (E1b-57K) (early transcription region 1B of 495 amino acid residues, with an approximate molecular weight of 57,000) protein of adenovirus 2 as encoded by the 495 amino acid open reading frame located in the adenovirus 2 DNA sequence between nucleotides 2016 and 3500. Additional proteins of 16,000 Mr and 18,000 Mr that are related to the E1B-495R protein were identified by cell-free translation of hybridization-selected mRNA. Analysis of [35S]methionine-containing amino terminal tryptic peptides by thin-layer chromatography showed that the E1B-495R, E1B-18K, and E1B-16K proteins all begin at the same initiation codon. The E1B-495R protein from 293 cells also has the same initial tryptic peptide, acetyl-methionyl-glutamyl-arginine. Sequence analysis of E1B-18K tryptic peptides indicated that this protein also has the same carboxy terminus as the E1B-495R protein and that it is derived from an mRNA that is spliced to remove sequences between nucleotides 2250 and 3269, resulting in a protein product of 155 amino acid residues. Analysis of E1B-16K tryptic peptides has not yet revealed the carboxy terminal structure of this protein. Both the E1B-495R and the E1B-155R (E1B-18K) proteins, as well as the E1B-16K protein, were precipitated from cell-free translations and from extracts of infected cells by antiserum against an amino terminal nonapeptide common to these proteins.

Peptide maps and N-terminal sequences of polypeptides from early region 1A of human adenovirus 5

Experiments exploring the reasons for a multiplicity of products from early region 1A of adenovirus 5 are described. Labeled early region 1A products from wild-type virus were synthesized in infected cells and in a cell-free system programmed with mRNA from infected cells, immunoprecipitated specifically with an antipeptide serum, E1A-C1, directed against the C-terminal sequence of E1A products, and separated by gel electrophoresis. Two-dimensional maps of [35S]methionine-labeled peptides were consistent with antigens of 52,000 daltons (52K) and 48.5K being from the 13S mRNA and antigens of 50K, 45K, and 35K from the 12S mRNA. Partial N-terminal sequences of 52K, 50K, 48.5K, and 45K synthesized in vitro showed that each of these antigens was initiated at the predicted ATG at nucleotide 560 in the DNA sequence. These results eliminate multiple initiation sites and proteolytic cleavage at the N-terminal end as sources of antigen diversity. Peptide maps and N-terminal sequences were obtained in a similar way for E1A products from the Ad5 deletion mutant dl1504, which lacks the normal initiator codon. As predicted, these polypeptides are initiated at the next ATG, 15 codons downstream in the wild-type sequence. These results are discussed in relation to Kozak's ribosomal scanning model.

Splice junctions in adenovirus 2 early region 4 mRNAs: multiple splice sites produce 18 to 24 RNAs

We localized the splice junctions in adenovirus 2 early region 4 (E4) mRNAs. Processing of the E4 precursor RNA positioned the donor splice site of the 5' leader sequence adjacent to acceptor sites near the 5' ends of five of the six open reading regions in the E4 transcription unit. Of particular interest among the E4 mRNAs is an extensively spliced class which includes multiple species with sizes ranging from 1.1 to 0.75 kilobases (kb). Purified 1.1- to 0.75-kb mRNAs specified at least 10 polypeptides in vitro. We detected eight acceptor and two donor splice sites utilized in the deletion of the intron from the 3' portion of these mRNAs. E4 RNAs were isolated from the cytoplasm of infected cells at 5, 9, 12, and 18 h after infection. The E4 mRNAs were present throughout infection, but different members of the 1.1- to 0.7-kb class were predominant at each time assayed. Alternate splicing of the 3.0-kb E4 precursor RNA can generate as many as 25 mRNAs that encode at least 16 polypeptides.

Adenovirus type 5 early region 1b gene product is required for efficient shutoff of host protein synthesis

To determine the role adenovirus 5 early region 1b-encoded 21- and 55-kilodalton proteins play in adenovirus productive infection, mutants have been isolated which were engineered to contain small deletions or insertions at 5.8, 7.9, or 9.6 map units. By using an overlap recombination procedure involving H5dl314 (delta 3.7 to 4.6 map units) DNA cleaved at 2.6 map units with ClaI and the adenovirus 5 XhoI-C (0 to 15.5 map units) fragment containing the desired mutation, viral mutants were isolated by their ability to produce plaques on KB cell line 18, which constitutively expresses only viral early region 1b functions (Babiss et al., J. Virol. 46:454-465, 1983). DNA sequence analysis of the viral mutants isolated (H5dl118, H5dl110, H5in127, and H5dl163) indicates that all of the viruses contain mutations which affect the 55-kilodalton protein, whereas dl118 should also produce a truncated form of the 21-kilodalton protein. When analyzed for their replication characteristics in HeLa cells, all of the mutant viruses exhibited extended eclipse periods and effected yields that were reduced to 10% or less of that produced by H5sub309 (parent virus of the mutants which is phenotypically identical to wild-type adenovirus 5). When compared with characteristics of sub309, the early and late transcription and DNA replication of the mutants were similar, but synthesis of late polypeptides and late cytoplasmic mRNAs was greatly reduced. Quantitation of mutant virus-specific late mRNAs associated with polysomes revealed a threefold reduction when compared with that of sub309. Analysis of infected cell extracts further revealed that these mutants were incapable of efficiently shutting off host cell protein synthesis, suggesting that the 55-kilodalton protein plays a role in this process. These data suggest that early region 1b products may function by interacting with additional viral or host cell macromolecules to modulate host cell shutoff or that some late viral mRNA or polypeptide may potentiate this reaction.

Antibody directed to a synthetic peptide encoding the NH2-terminal 16 amino acids of the adenovirus type 2 E1B-53K tumor antigen recognizes the E1B-20K tumor antigen

A peptide, H2N-Glu-Arg-Arg-Asn-Pro-Ser-Glu-Arg-Gly-Val-Pro-Ala-Gly-Phe-Ser-Gly-(Cys )COOH, containing the amino acid sequence at the NH2 terminus of the adenovirus type 2 (Ad2) E1B-coded large T antigen (E1B-53K) has been synthesized. Anti-peptide antibody was generated in rabbits and used to immunoprecipitate Ad T antigens from Ad2 early infected cell extracts. In addition to the expected E1B-53K T antigen, anti-peptide antibody precipitated the Ad2 E1B-20K T antigen that was previously shown to be related to E1B-53K (M. Green, K.H. Brackmann, M.A. Cartas, and T. Matsuo, J. Virol. 42, 30-41, 1982). Anti-peptide prepared against the COOH terminus of the E1B-53K T antigen or against the NH2 terminus of the E1B-19K T antigen did not precipitate the E1B-20K T antigen. These data suggest that the Ad2 E1B-20K T antigen initiates translation at nucleotide 2016 in reading frame 3, as does E1B-53K. The viral mRNA that encodes the E1B-20K T antigen has not been identified.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of adenovirus transforming proteins from early regions 1A and 1B with antisera to inducible fusion antigens produced in Escherichia coli

Plasmid vectors were constructed which expressed three adenovirus tumor antigens fused to a portion of the trpE protein of Escherichia coli. Insertion of adenovirus type 2 DNA from early region 1A (E1A) into such a plasmid led to a fusion protein which contained the C-terminal 266 amino acids of the 289-amino acid protein encoded by the viral 13S mRNA. Similarly, insertion of adenovirus type 5 DNA corresponding to the E1B 55- and 21-kilodalton proteins led to production of fusion proteins containing amino acid sequences from these proteins. After induction with indoleacrylic acid, fusion proteins accumulated stably in the E. coli cells. By using a simple extraction of insoluble protein, 1 to 10 mg of fusion protein per liter of culture was obtained. The fusion proteins were purified on preparative polyacrylamide gels and used to immunize rabbits. Specific antisera for the E1A 289- and closely related 243-amino acid proteins and the E1B 55- and 21-kilodalton proteins were obtained. These sera were used to immunoprecipitate the tumor antigens in cells infected with wild-type and various mutants of adenovirus or to analyze them by an immunoblotting procedure. Mutant E1A proteins in which the C-terminal 70 amino acids are deleted were phosphorylated to much lower extents than the wild-type E1A proteins. This indicates that the deleted region is important for the process of phosphorylation. The E1A proteins were extracted, sedimented in glycerol gradients, analyzed by immunoprecipitation, and found to sediment primarily as monomers.

Establishment and characterization of hamster cell lines transformed by restriction endonuclease fragments of adenovirus 5

We have established a library of hamster cells transformed by adenovirus 5 DNA fragments comprising all (XhoI-C, 0 to 16 map units) or only a part (HindIII-G, 0 to 7.8 map units) of early region 1 (E1: 0 to 11.2 map units). These lines have been analyzed in terms of content of viral DNA, expression of E1 antigens, and capacity to induce tumors in hamsters. All cells tested were found to express up to eight proteins encoded within E1A (0 to 4.5 map units) with apparent molecular weights between 52,000 (52K) and 25K. Both G and C fragment-transformed lines expressed a 19K antigen encoded within E1B (4.5 to 11.2 map units), whereas an E1B 58K protein was detected in C fragment-transformed, but not G-fragment-transformed, lines. No clear distinction could be drawn between cells transformed by HindIII-G and by XhoI-C in terms of morphology or tumorigenicity, suggesting that the E1B 58K antigen plays no major role in the maintenance of oncogenic transformation, although possible involvement of truncated forms of 58K cannot be ruled out. Sera were collected from tumor-bearing animals and examined for ability to immunoprecipitate proteins from infected cells. The relative avidity of sera for different proteins was characteristic of the cell line used for tumor induction, and the specificity generally reflected the array of viral proteins expressed by the corresponding transformed cells. However, one notable observation was that even though all transformed lines examined expressed antigens encoded by both the 1.1- and 0.9-kilobase mRNAs transcribed from E1A, tumor sera made against these lines only precipitated products of the 1.1-kilobase message. Thus, two families of E1A proteins, highly related in terms of primary amino acid sequence, appear to be immunologically quite distinct.

Human adenovirus 2 E1B-19K and E1B-53K tumor antigens: antipeptide antibodies targeted to the NH2 and COOH termini

The human adenovirus 2 (Ad2) transforming region is located in the left 11.1% of the viral genome and encodes two early transcription units, E1A and E1B. Based on the amino acid sequence deduced from the Ad2 E1B DNA sequence (Gingeras et al., J. Biol. Chem. 257:13475-13491, 1982), we have prepared antibodies against synthetic peptides, 8 to 16 amino acids in length, encoded at the NH2 and COOH termini of the major E1B-19K and E1B-53K tumor antigens. The antipeptide antibodies immunoprecipitated the targeted E1B-19K or E1B-53K tumor antigens from extracts of Ad2-infected cells. The specificity of the peptide competition studies. Antipeptide antibodies directed to the NH2 and COOH termini immunoprecipitated the E1B-19K and E1B-53K tumor antigens from two Ad2-transformed rat cell lines, F17 and F4, providing evidence that identical tumor antigens are synthesized in Ad2-infected and Ad2-transformed cells. These results show that the E1B-19K and E1B-53K T antigens are not processed proteolytically at either the NH2 or COOH terminus. Our data provide strong evidence at the protein level that the E1B-19K and E1B-53K tumor antigens partially overlap in DNA sequence, with the E1B-19K initiating translation at the first ATG at nucleotide 1711 in translation reading frame 1 and the E1B-53K tumor antigen initiating translation at the second ATG at nucleotide 2016 in reading frame 3. This confirms the results of others on the N-terminal amino acid sequence of E1B-19K and theoretical deductions based on the DNA sequence. Our findings prove that the large E1B-53K T antigen initiates translation at the second ATG at nucleotide 2016 and not at equally plausible initiation codons located farther downstream at nucleotides 2202 and 2235. Thus, the E1B-53K T antigen is another example of a protein which initiates translation at an internal ATG rather than at the 5'-proximal ATG.

The pattern of integration of viral DNA sequences in the adenovirus 5-transformed human cell line 293

The pattern of integrated adenovirus 5 DNA in the adenovirus 5-transformed human cell line 293 was analyzed by DNA blot hybridization. In contrast to a previous report, only sequences from the left end of the genome were detected. The viral DNA was contained in a unique DNA fragment generated by cleavage of 293 DNA with either EcoRI or BamHI, two enzymes which do not cut the integrated viral DNA. Further mapping studies indicated that the integrated sequence was probably colinear with viral DNA. The joining to host cell sequences occurred between viral nucleotide base pairs 1 and 270 (a site for BalI) on one end and 4123 (SmaI site) and 5372 (BalI site) at the other end. The viral DNA was not tandemly repeated. These results suggest that integration of viral sequences into the host genome probably occurred at a single site. If any subsequent duplications of viral DNA took place, then duplication of extensive cellular sequences also must have occurred.

Intracellular localization of adenovirus type 5 tumor antigens in productively infected cells

The intracellular localization of tumor antigens of human adenovirus type 5 (Ad5) during lytic infection of KB cells has been studied. The cells were pulse labeled with [35S]methionine early after infection and early proteins of 58,000 D (58K), 44K, 19K, 18.5K, and 14K detectable by immunoprecipitation with hamster antitumor serum were assayed for association with cytoplasm, nucleoplasm, chromatin, cytosol, cytoskeleton, and membranes. The 44,000 D (44K) tumor antigen encoded in early region 1A (E1A: 0-4.4%) was recovered in approximately equal amounts from cytoplasmic and nucleoplasmic fractions of pulse-labeled cells and within the cytoplasmic compartment was found in the cytosol as well as associated with the cytoskeleton. The E1B-58K (E1B: 4.5-11.2%) antigen was also found to be associated with the cytoplasmic and nucleoplasmic fractions in approximately equal amounts but unlike the E1A-44K showed no affinity for cytoskeletons. Pulse-chase and immunofluorescence experiments suggested the 58K antigen accumulated in the nucleus late in infection. The E1B-19K antigen was found almost exclusively associated with the membrane fraction of infected KB cells and was resolved in polyacrylamide gels into two related species of 18.5K and 19K. Immunofluorescence studies on the E1B 18.5-19K doublet suggested that within a population of infected HeLa cells a small minority seemed to be expressing copious amounts of stainable antigen. Cell fractionation and immunofluorescence studies showed that the E4-14K antigen was a nuclear protein and the only antigen in this study which showed a significant association with a nuclear subfraction composed almost entirely of histones. The implications of these findings for the roles of the Ad5 tumor antigens in lytic infection and transformation are discussed.

Localization of the adenovirus E1Aa protein, a positive-acting transcriptional factor, in infected cells infected cells

The function of the adenovirus E1Aa protein (the product of the 13S E1A mRNA) during a productive viral infection is to activate transcription of the six early viral transcription units. To study the mechanism of action of this protein, a peptide which was 13 amino acids long and had a sequence unique to the protein product of the adenovirus 13S E1A mRNA (pE1Aa) was coupled to keyhole limpet hemocyanin and used to raise an antibody in rabbits. The resulting antiserum was specific to this protein and did not react with the protein product of the 12S E1A mRNA, which shares considerable sequence with the E1Aa protein. This antiserum was used to probe for the E1Aa protein in situ by indirect immunofluorescence and in extracts of infected HeLa cells. We found that the protein was associated with large cellular structures both in the nucleus and in the cytoplasm. The nuclear form of the protein was analyzed further and was found to purify with the nuclear matrix.

Transformation of rodent cells by DNA extracted from transformation-defective adenovirus mutants

Complementation group II host range mutants of adenovirus type 5 which map in early region 1B (E1B, 4.5 to 11.0 map units) have been shown to be defective for the synthesis of the E1B 58,000-dalton (58K) antigen in infections of HeLa or KB cells (Lassam et al., Cell 18:781-791, 1979) and unable to transform cultured rodent cells (Graham et al., Virology 86:10-21, 1978). In this report we show that DNA extracted from group II mutants hr6 and hr50 can transform rat cells with the same efficiency as wild-type DNA. Furthermore, group II mutant-transformed hamster cells were shown to contain no detectable E1B 58K tumor antigen but were capable of inducing tumors in newborn hamsters. Hamster cell lines 1019-3 and 1019-C3, transformed by hr50 DNA, produced no detectable quantities of either the E1B 58K or 19K antigen but nonetheless exhibited a fully transformed oncogenic phenotype. Our results show that the E1B 58K antigen is not absolutely required for oncogenic transformation and suggest that even cells lacking the 19K protein can be oncogenic.

Identification of human adenovirus early region 1 products by using antisera against synthetic peptides corresponding to the predicted carboxy termini

Synthetic peptides were prepared which corresponded to the carboxy termini of the human adenovirus type 5 early region 1B (E1B) 58,000-molecular-weight (58K) protein (Tyr-Ser-Asp-Glu-Asp-Thr-Asp) and of the E1A gene products (Tyr-Gly-Lys-Arg-Pro-Arg-Pro). Antisera raised against these peptides precipitated polypeptides from adenovirus type 5-infected KB cells; serum raised against the 58K carboxy terminus was active against the E1B 58K phosphoprotein, whereas serum raised against the E1A peptide immunoprecipitated four major and at least two minor polypeptides. These latter proteins migrated with apparent molecular weights of 52K, 50K, 48.5K, 45K, 37.5K, and 35K, and all were phosphoproteins. By using tryptic phosphopeptide analysis, the four major species (52K, 50K, 48.5K, and 45K) were found to be related, as would be expected if all were products of the E1A region. The ability of the antipeptide sera to precipitate these viral proteins thus confirmed that the previously proposed sequence of E1 DNA and mRNA and the reading frame of the mRNA are correct. Immunofluorescent-antibody staining with the antipeptide sera indicated that the 58K E1B protein was localized both in the nucleus and in the cytoplasm, especially in the perinuclear region. The E1A-specific serum also stained both discrete patches in the nucleus and diffuse areas of the cytoplasm. These data suggest that both the 58K protein and the E1A proteins may function in or around the nucleus. These highly specific antipeptide sera should allow for a more complete identification and characterization of these important viral proteins.

Localization of the adenovirus E1Aa protein, a positive-acting transcriptional factor, in infected cells infected cells

The function of the adenovirus E1Aa protein (the product of the 13S E1A mRNA) during a productive viral infection is to activate transcription of the six early viral transcription units. To study the mechanism of action of this protein, a peptide which was 13 amino acids long and had a sequence unique to the protein product of the adenovirus 13S E1A mRNA (pE1Aa) was coupled to keyhole limpet hemocyanin and used to raise an antibody in rabbits. The resulting antiserum was specific to this protein and did not react with the protein product of the 12S E1A mRNA, which shares considerable sequence with the E1Aa protein. This antiserum was used to probe for the E1Aa protein in situ by indirect immunofluorescence and in extracts of infected HeLa cells. We found that the protein was associated with large cellular structures both in the nucleus and in the cytoplasm. The nuclear form of the protein was analyzed further and was found to purify with the nuclear matrix.

Transformation of rat cells by cyt mutants of adenovirus type 12 and mutants of adenovirus type 5

Several mutants with much reduced oncogenicity (spontaneous mutants H12 cyt 52 and H12 cyt 70 and UV-induced mutants H12 cyt 61, H12 cyt 62, and H12 cyt 68) of the highly oncogenic adenovirus type 12 (Ad12) were studied for their ability to transform primary baby rat kidney cells. Four of the mutants showed much reduced capacity to transform cells in vitro, while H12 cyt 61 transformed cells as efficiently as the wild-type virus. Viral gene expression in several cell lines established from cultures infected by cyt mutants was studied, and it was found that viral sequences belonging to the left 16% of Ad12 were always transcribed. These results suggest that the function of the transformed state is not defective in the cyt mutants studied. Heterotypic complementation studies showed that the defect(s) in a cyt mutant can be corrected by an Ad7 function. Ad5 dl 313, with a deletion between 3.5 and 10.5 map units, transformed rat cells only at high multiplicity. These results suggest that the region E1B of adenoviruses may be required for efficient transformation of rat cells.

Expression of adenovirus-2 early region 4: assignment of the early region 4 polypeptides to their respective mRNAs, using in vitro translation

Adenovirus-2 early region 4 (E4; map positions 91.3 to 99.1) encodes six 5' and 3' coterminal, differently spliced mRNAs, which are 2.5, 2.1, 1.8, 1.5, 1.2, and 0.8 kilobases (kb) long. Hybridization selection with five cloned viral DNA fragments that hybridize with subsets of E4 mRNAs was used to purify these six mRNAs and a previously unreported 3.0-kb mRNA from virus-infected cells. E4 mRNAs which were purified by hybridization selection with cloned EcoRI fragment C (map positions 89.7 to 100) were also fractionated by size. The purified mRNAs were then translated in rabbit reticulocyte or wheat germ lysate systems. The full complement of E4 mRNAs specified as many as 16 different polypeptides, with molecular weights ranging from 24,000 (24K) to 10K. The most abundant E4 mRNA, which was 2.1 kb long, specified an 11K polypeptide. The 1.5-kb mRNA, which differed from the 2.1-kb mRNA only by deletion of a second intron from the 3' untranslated region, also specified an 11K polypeptide. The second most abundant mRNA, which was 1.8 kb long, and the 1.2-kb mRNA, which had an intron deleted from the 3' untranslated region, specified a 15K polypeptide. This polypeptide was labeled more intensely with [5,6-(3)H]leucine than with [35S]methionine. The 3.0- and 2.5-kb mRNAs specified four polypeptides (24K, 22K, 19K, and 17K). Translation of E4 mRNAs with a mean size of 0.8 kb, which accumulated preferentially in the presence of cycloheximide, yielded at least 10 polypeptides that migrated in polyacrylamide gels with apparent molecular weights ranging from 21,800 to 10,000. On the basis of translation in wheat germ lysates and the distribution of polypeptides encoded by size-fractionated mRNAs, we concluded that the 0.8-kb mRNA size class includes a heterogeneous mixture of mRNAs which are probably formed as the result of utilization of alternate splice acceptor and donor sites during removal of the second intron. Our polypeptide assignments for the 2.1-, 1.8-, 1.5-, and 1.2-kb mRNAs are compatible with locations of two open coding regions in the DNA sequence (Herisse et al., Nucleic Acids Res. 9:4023-4042, 1981). The relationship between the four polypeptides encoded by the 3.0- and 2.5-kb mRNAs and the two open coding regions is discussed. The production of multiple polypeptides from a heterogenous mixture of mRNAs in the 0.8-kb size class is compatible with two large open coding regions in that part of the sequence. Thus, nearly all of the potential coding information in the leftward strand of E4 is expressed in translatable form during infection. Moreover, alternate splicing of the 0.8-kb mRNA size class can produce multiple polypeptides with common amino-terminal and different carboxy-terminal amino acid sequences, which may have the same function but different specificities.