Adenovirus early region 4 is required for efficient virus particle assembly

The Interplay between Adeno-Associated Virus and its Helper Viruses

The adeno-associated virus (AAV) is a small, nonpathogenic parvovirus, which depends on helper factors to replicate. Those helper factors can be provided by coinfecting helper viruses such as adenoviruses, herpesviruses, or papillomaviruses. We review the basic biology of AAV and its most-studied helper viruses, adenovirus type 5 (AdV5) and herpes simplex virus type 1 (HSV-1). We further outline the direct and indirect interactions of AAV with those and additional helper viruses.

Adenoviral E4 34K protein interacts with virus packaging components and may serve as the putative portal

Studies on dsDNA bacteriophages have revealed that a DNA packaging complex assembles at a special vertex called the 'portal vertex' and consists of a portal, a DNA packaging ATPase and other components. AdV protein IVa2 is presumed to function as a DNA packaging ATPase. However, a protein that functions as a portal is not yet identified in AdVs. To identify the AdV portal, we performed secondary structure analysis on a set of AdV proteins and compared them with the clip region of the portal proteins of bacteriophages phi29, SPP1 and T4. Our analysis revealed that the E4 34K protein of HAdV-C5 contains a region of strong similarity with the clip region of the known portal proteins. E4 34K was found to be present in empty as well as mature AdV particles. In addition, E4 34K co-immunoprecipitates and colocalizes with AdV packaging proteins. Immunogold electron microscopy demonstrated that E4 34K is located at a single site on the virus surface. Finally, tertiary structure prediction of E4 34K and its comparison with that of single subunits of Phi29, SPP1 and T4 portal proteins revealed remarkable similarity. In conclusion, our results suggest that E4 34K is the putative AdV portal protein.

Oncolytic virotherapy of breast cancer

The use of replication competent viruses that selectively target and destroy cancer cells has rapidly evolved over the past decade and numerous innovative oncolytic viruses have been created. Many of these promising anti-cancer agents have recently entered into clinical trials (including those on breast cancer) and demonstrated encouraging safety and efficacy. Virotherapeutic strategies are thus of considerable interest to combat breast cancer in both (i) the primary disease situation in which relapse should be avoided as good as possible and (ii) in the metastatic situation which remains incurable to date. Here, we summarize data from preclinical and clinical trials using oncolytic virotherapy to treat breast cancer. This includes strategies to specifically target breast cancer cells, to arm oncolytic viruses with additional therapeutic transgenes and an outlining of future challenges when translating these promising therapeutics "from bench to bedside".

Transcription mapping and characterization of proteins produced from early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus 3 (PAdV-3) was characterized by Northern blot, rapid amplification of cDNA ends (RACE), RT-PCR and cDNA sequence analysis. Northern blot analysis revealed three different classes of transcripts, which appeared and peaked at different times post-infection. The RT-PCR, RACE and cDNA sequence analysis identified nine major E4 transcripts, all of which shared a 107-bp 5' leader sequence and a 126-bp 3' terminus. These transcripts have one to three introns removed. Interestingly, of the nine major transcripts, there was one fusion transcript of ORFp1 and ORFp7 (ORFp1/7), which codes for a protein of 119 amino acids. All transcripts initiated at nucleotide 33740 of the PAdV-3 genome. To identify proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding p2, p3, p4 or p7 proteins. Serum against p2, p3 and p4 immunoprecipitated proteins of 13.5, 13.6 and 15.3 kDa, respectively, in in-vitro transcribed and translated mRNA and in PAdV-3-infected cells. Serum against p7 immunoprecipitated a protein of 19.8 kDa in in-vitro transcription and translation analysis but recognized two proteins of 19.8 kDa (encoded by ORFp7) and 14 kDa (encoded by the fusion transcript ORF1/7) in PAdV-3-infected cells. The protein encoded by ORFp2 was localized in the nucleus of PAdV-3-infected cells. The proteins encoded by ORFp3 and ORFp7\ORFp1/7 were detected in the cytoplasm of PAdV-3-infected cells. However, the protein encoded by ORFp4 was observed both in the cytoplasm and nucleus of PAdV-3-infected cells.

Inactivating intracellular antiviral responses during adenovirus infection

DNA viruses promote cell cycle progression, stimulate unscheduled DNA synthesis, and present the cell with an extraordinary amount of exogenous DNA. These insults elicit vigorous responses mediated by cellular factors that govern cellular homeostasis. To ensure productive infection, adenovirus has developed means to inactivate these intracellular antiviral responses. Among the challenges to the host cell is the viral DNA genome, which is viewed as DNA damage and elicits a cellular response to inhibit replication. Adenovirus therefore encodes proteins that dismantle the cellular DNA damage machinery. Studying virus-host interactions has yielded insights into the molecular functioning of fundamental cellular mechanisms. In addition, it has suggested ways that viral cytotoxicity can be exploited to offer a selective means of restricted growth in tumor cells as a therapy against cancer. In this review, we discuss aspects of the intracellular response that are unique to adenovirus infection and how adenoviral proteins produced from the early region E4 act to neutralize antiviral defenses, with a particular focus on DNA damage signaling.

Immune responses against adenoviral vectors and their transgene products: a review of strategies for evasion

Human adenoviruses have been adopted as attractive vectors for in vivo gene therapy since they have a well-characterized genomic organization, can be grown to high titres and efficiently transduce a wide spectrum of dividing and non-dividing cells. However, the first-generation of adenoviral (Ad) vectors yielded only transient expression of the transgene in most immunocompetent mice. This constituted a major limitation of this early vector type. In contrast, persistent transgene expression can be established in immunodeficient mice. This suggests that the immunogenicity of adenoviral vectors limits the effective period of adenovirus-based gene therapy. Much effort has been put in devising strategies to circumvent the limitations imposed onto gene therapy by the immune system. Improvements in vector design have significantly improved the performance of the adenovirus vectors. Based on these results it is reasonable to anticipate that new modifications of the vectors will overcome some of the immunological barriers and will further expand the applicability of adenovirus-derived vectors.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.

New oncolytic adenoviruses with hypoxia- and estrogen receptor-regulated replication

Oncolytic adenoviruses with restricted replication can be produced if the expression of crucial transcription units of the virus is controlled by tissue- or tumor-specific promoters. Here we describe a method for the rapid incorporation of exogenous promoters into the E1A and E4 regions of the human adenovirus type 5 genome. Using this system, we have generated AdEHT2 and AdEHE2F, two conditionally replicative adenoviruses for the treatment of breast cancer. The expression of the E1A gene in both viruses is controlled by a minimal dual-specificity promoter that responds to estrogens and hypoxia. The tight regulation of E1A expression correlated with the ability of these viruses to replicate and kill human cancer cells that express estrogen receptors, or are maintained under hypoxic conditions. The telomerase reverse transcriptase (TERT) promoter and the E2F-1 promoter are preferentially activated in cancer cells. They were introduced into the E4 region of AdEHT2 and AdEHE2F, respectively. The telomerase core promoter failed to block the replication of the virus in telomerase-negative cells. In contrast, AdEHE2F was attenuated in nontransformed quiescent cells growing under normoxic conditions, suggesting that an intact pRB pathway with low levels of E2F transcription factors acts as a negative modulator for the virus. These data indicate that the simultaneous regulation of E1A and E4 viral transcription units by the appropriate combination of promoters can increase the tumor selectivity of oncolytic adenoviruses.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.

A novel, conditionally replicative adenovirus for the treatment of breast cancer that allows controlled replication of E1a-deleted adenoviral vectors

The efficiency of gene therapy strategies against cancer is limited by the poor distribution of the vectors in the malignant tissues. To solve this problem, a new generation of tumor-specific, conditionally replicative adenoviruses is being developed. To direct the replication of the virus to breast cancer, we have considered one characteristic present in a great proportion of these cancers, which is the expression of estrogen receptors (ERs). On the basis of the wild-type adenovirus type 5, we have constructed a conditionally replicative adenovirus (Ad5ERE2) in which the E1a and E4 promoters have been replaced by a portion of the pS2 promoter containing two estrogen-responsive elements (EREs). This promoter induces transcriptional activation of the E1a and E4 units in response to estrogens in cells that express the ERs. Ad5ERE2 is able to kill ER(+) human breast cancer cell lines as efficiently as the wild-type virus, but has decreased capacity to affect ER(-) cells. By complementation of the E1a protein in trans, Ad5ERE2 allows restricted replication of a conventional E1a-deleted adenoviral vector. When a virus expressing the proapoptotic gene Bc1-xs (Clarke et al., Proc. Natl. Acad. Sci. U.S.A. 1995;92:11024-11028) is used in combination with Ad5ERE2, the ability of both viruses to induce cell death is dramatically increased, and the effect can be modulated by addition of the antiestrogen tamoxifen.

Reduced toxicity, attenuated immunogenicity and efficient mediation of human p53 gene expression in vivo by an adenovirus vector with deleted E1-E3 and inactivated E4 by GAL4-TATA promoter replacement

A recombinant adenovirus with deleted E1 and E3, and E4-inactivated by replacing the E4 promoter with a synthetic promoter composed of a minimal TATA box and five consensus yeast GAL4-binding site elements was developed and used to express the human tumor suppresser gene p53. The toxicity and immunogenicity of this vector and vector-mediated p53 gene expression in vivo were studied in immunocompetent C3H and C57BL/6 mice. Expression of the late viral gene product, hexon protein, was observed in C3H and C57BL/6 mice injected with E4 wild-type adenovirus constructs Adv-cmv-beta-Gal (BG), Adv-cmv-hp53 (WT), and empty E1- vector Adv-E4 (EW) 3 to 28 days after injection, but was undetectable in mice treated with E4 modified empty E1- vector Adv-GAL4 (EG) or Adv-cmv-hp53-GAL4 (G4). Expression of the p53 gene was observed in both WT- and G4-injected C3H and C57BL/6 mouse livers from days 3 to 28. Ten weeks after injection, p53 gene expression was still detected in G4-treated C57BL/6 mice at similar levels, but was not detectable in WT-treated mice. Vector-induced liver toxicity was evaluated by analyzing serum transaminases (SGOT and SGPT) activities. In all cases, SGOT and SGPT activities were markedly decreased in EG-treated C3H and C57BL/6 mice compared with those in EW-treated mice on days 3, 7 and 14 after injection. In C57BL/6 mice, the total anti-adenoviral CTL activities were two- to three-fold higher in animals treated with EW vector than in those treated with EG vector. These results suggest that inactivation of the E4 promoter efficiently diminished the viral replication and the late viral gene expression, reduced host immune response and consequently reduced toxicity and prolonged the duration of transgene expression in vivo.

Fiberless recombinant adenoviruses: virus maturation and infectivity in the absence of fiber

In vivo targeting of therapeutic genes to specific tissues has become a major issue in gene therapy, in particular when recombinant adenovirus vectors are used. Restriction of the viral tropism to selected cell types requires the abrogation of the interaction between the viral fiber and its natural cellular receptors and the introduction of a new binding specificity into the virion. In this context, fiberless adenoviruses are attractive vectors, since they may be used as substrates for the insertion of a new ligand in other capsid proteins. In this study, we confirm by using cloned full-length adenovirus genomes with the fiber gene deleted that efficient virus particle formation can occur in the absence of fiber. As expected, the infectivity of such fiberless viruses was severely reduced, but it could be only partially restored when the viruses were produced in cells stably providing the fiber in trans. Although incorporation of penton base into the fiberless particles was normal and binding of the particles to the cellular integrins was functional, several pieces of experimental evidence suggest that later steps in the cell entry process are impaired in correlation with an incorrect maturation of several structural proteins of the fiberless particles. These observations support the hypothesis that the fiber protein may have additional biological functions besides its role in cell binding. Together with the fiber complementation cells, such fiberless vectors constitute unique tools to investigate the role of the fiber in virus assembly, maturation, and cell entry and to explore the possibility of deriving gene transfer vectors with novel target specificities.

Long-term gene delivery into the livers of immunocompetent mice with E1/E4-defective adenoviruses

We have compared the in vitro and in vivo behaviors of a set of isogenic E1- and E1/E4-defective adenoviruses expressing the lacZ gene of Escherichia coli from the Rous sarcoma virus long terminal repeat. Infection of tumor-derived established cell lines of human origin with the doubly defective adenoviruses resulted in (i) a lower replication of the viral backbone that correlated with reduced levels of E2A-specific RNA and protein, (ii) a significant shutoff of late gene and protein expression, and (iii) no apparent virus-induced cytotoxicity. Independently of the extent of the deletion, the additional inactivation of E4 from the viral backbone therefore drastically disabled the virus in vitro, with no apparent effect on transgene expression. A lacZ-transgenic model was used to compare the different recombinant adenoviruses in the livers of C57BL/6 mice. The immune response to the virally encoded beta-galactosidase was minimal in this model, as infusion of the E1-defective adenovirus resulted in a time course of transgene expression that mimicked that in immunodeficient (nu/nu) mice, with very little inflammation and necrosis in the liver. Administration of a doubly defective adenovirus to the transgenic animals led to long-term extrachromosomal persistence of viral DNA in the liver, with no detectable methylation of CpG dinucleotides. However, transient transgene expression was observed independently of the extent of the E4 deletion, suggesting that the choice of the promoter may be critical to maintain transgene expression from these attenuated adenovirus vectors.

Efficient dual transcomplementation of adenovirus E1 and E4 regions from a 293-derived cell line expressing a minimal E4 functional unit

Transgene expression after the administration of recombinant adenovirus with E1 deleted is constantly transient. It is admitted that E1A-substituting activities of cellular or viral origin allow viral antigen synthesis and trigger cytotoxic lymphocyte-mediated clearance of the recipient cells. Our approach to solving this problem relies on the additional deletion of the E4 region from the vector backbone as this region upregulates viral gene expression at both transcriptional and posttranscriptional levels. As a prerequisite to the construction of E1 E4 doubly defective adenoviruses, we investigated the possibility of transcomplementing both functions within a single cell. In particular, the distal ORF6+ORF7 segment from the E4 locus of adenovirus type 5 was cloned under the control of the dexamethasone-inducible mouse mammary tumor virus long terminal repeat. Following transfection into 293 cells, clone IGRP2 was retained and characterized as it can rescue the growth defect of all E1+ E4- adenoviral deletants tested. DNA and RNA analysis experiments verified that the mouse mammary tumor virus promoter drives the expression of the ORF6+ORF7 unit and permits its bona fide alternative splicing, generating ORF6/7 mRNA in addition to the ORF6-expressing primary transcript. Importantly, IGRP2 cells sustain cell confluence for a period longer than that of 293 parental cells and allow the plaque purification of E1- or E4- defective viruses. The dual expression of E1 and E4 regulatory genes within IGRP2 cells is demonstrated by the construction, plaque purification, and helper-free propagation of recombinant lacZ-encoding doubly defective adenoviruses harboring different E4 deletions. In addition, the emergence, if any, of replicative particles during viral propagation in this novel packaging cell line will be drastically impaired as only a limited segment of E4 has been integrated.

Gene therapy for brain tumors

Gene therapy has opened new doors for treatment of neoplastic diseases. This new approach seems very attractive, especially for glioblastomas, since treatment of these brain tumors has failed using conventional therapy regimens. Many different modes of gene therapy for brain tumors have been tested in culture and in vivo. Many of these approaches are based on previously established anti-neoplastic principles, like prodrug activating enzymes, inhibition of tumor neovascularization, and enhancement of the normally weak anti-tumor immune response. Delivery of genes to tumor cells has been mediated by a number of viral and synthetic vectors. The most widely used paradigm is based on the activation of ganciclovir to a cytotoxic compound by a viral enzyme, thymidine kinase, which is expressed by tumor cells, after the gene has been introduced by a retroviral vector. This paradigm has proven to be a potent therapy with minimal side effects in several rodent brain tumor models, and has proceeded to phase 1 clinical trials. In this review, current gene therapy strategies and vector systems for treatment of brain tumors will be described and discussed in light of further developments needed to make this new treatment modality clinically efficacious.

Defective synthesis of early region 4 mRNAs during abortive adenovirus infections in monkey cells

Human adenovirus 2 grows poorly in monkey cells, partly because of defects in late gene expression. Since deletions in early region 4 (E4) cause similar defects in late gene expression, we examined E4 mRNA expression in abortive infections. Processing of E4 mRNAs was defective during abortive infections, most likely at the level of splicing. At early times in productive infections in HeLa cells, the major E4 species produced is a 2-kb mRNA; at late times, a shift occurs so that smaller spliced E4 mRNAs are also produced. In CV-1 cells, a nonpermissive monkey cell line, this shift did not take place and only the 2-kb species was produced at late times, suggesting a defect in E4 mRNA splicing during abortive infections. The adenovirus DNA-binding protein (DBP) was required for normal processing of E4 mRNAs, since a host range mutant (hr602) containing an altered DBP gene showed a normal late E4 mRNA pattern in CV-1 cells; in addition, DBP was required during infections in HeLa cells for late E4 mRNA expression. DBP was not required for production of the late E4 pattern in transient expression assays in HeLa or 293 cells, suggesting that a second factor in addition to the DBP, present during infection but not transfection, modulates E4 mRNA processing.

Adenovirus early region 4 stimulates mRNA accumulation via 5' introns

The adenovirus major late transcription unit accounts for most virus-specific transcription late after infection. All mRNAs expressed from this unit carry a short spliced leader, the so-called tripartite leader, attached to their 5' ends. Here we describe a function for an adenovirus gene product in the control of major late mRNA abundance. We show that early region 4 (E4) stimulates mRNA accumulation from tripartite leader intron-containing transcription units approximately 10-fold in short-term transfection assays. The effect was already detectable in nuclear RNA and was not due to a transcriptional activation through any of the major late promoter elements or through an effect at nuclear to cytoplasmic mRNA transport. A surprising positional effect of the intron was noted. To be E4 responsive, the intron had to be placed close to the pre-mRNA 5' end. The same intron located far downstream in the 3' untranslated region of the mRNA was not E4 responsive. The E4 enhancement was not dependent on specific virus exon or intron sequences. These results suggest that E4 modulates a general pathway in mammalian mRNA formation.

Interaction of adenoviral E4 and E1b products in late gene expression

The adenovirus 294R protein of E4 ORF 6 forms a physical and functional complex with the 496R protein product of E1b. The E4 294R ORF 6 protein also functions in parallel with the E4 116R ORF 3 protein in viral late protein synthesis. We have examined the roles of these three proteins and the protein complex in viral late protein synthesis and late message metabolism, by comparing the phenotypes of E4 294R-, E4 116R-, and E1b 496R- mutants to those of a series of double mutants. Our data indicate that the 294R and 116R proteins act in parallel to permit the accumulation of normal levels of unprocessed late viral RNA in the nucleus of infected cells. Both 294R and 496R function in parallel with the 116R protein in viral nuclear RNA accumulation, but do so to different degrees, suggesting that 294R and 496R may have roles apart from the functional complex in mediating accumulation of viral messages in the nucleus, or that they have nonequivalent roles within the complex. Our results are also consistent with a role for the 496R/294R protein complex in mediating efficient transport of late messages from the nucleus to the cytoplasm and/or in maintaining the stability of those messages on reaching the cytoplasm, as suggested previously (S. Pilder, M. Moore, J. Logan, and T. Shenk, 1986, Mol. Cell. Biol. 6, 470-476). Finally the 116R protein seems to act in parallel with the complex to permit normal viral DNA replication.

Adenovirus early region 4 encodes two gene products with redundant effects in lytic infection

In order to assign specific functions to individual gene products encoded by adenovirus type 5 early region 4 (E4), we have constructed and analyzed a set of mutant viruses that express individual E4 open reading frames or combinations of open reading frames. The results of these analyses demonstrate that the gene products of E4 open reading frames 3 and 6 have redundant effects in viral lytic infection. These E4 products independently augment viral DNA replication, viral late protein synthesis, the shutoff of host cell protein synthesis, and the production of infectious virus. The product of open reading frame 6 is more efficient in the regulation of these processes than is the product of open reading frame 3. The regulation of viral DNA replication and the control of viral and cellular protein synthesis appear to be separable functions associated with both E4 gene products. The role of early region 4 in adeno-associated virus helper function, however, is mediated only by the product of open reading frame 6. Finally, we demonstrate that E4 mutant viruses display a multiplicity-leakiness phenotype which is consistent with the regulatory role that this region plays in viral infection.

Complementation of adenovirus E4 mutants by transient expression of E4 cDNA and deletion plasmids

Human adenovirus mutants that carry a large deletion in early region 4 (E4) are severely defective in the synthesis of viral late proteins. Plasmids that carry intact E4 sequences can complement the late protein synthetic defect of such mutants when introduced into infected cells by transfection, presumably due to the transient expression of E4 products. Cells transfected with cDNA clones capable of expressing E4 open reading frame (ORF) 6, or deletion mutant clones expected to express either E4 ORF 6 or E4 ORF 3, also complement the mutants' defects. Thus, these E4 ORFs can individually satisfy the requirement for E4 products in viral late gene expression, and function effectively in the absence of other E4 products. Some E4 deletion mutants also exhibit a defect in the production of viral DNA. All of the clones that stimulate late gene expression also enhance one such mutant's ability to accumulate viral DNA. Thus, the ORF 3 and ORF 6 products are also individually sufficient to provide an E4 function necessary for normal viral DNA replication in the absence of other E4 products.

Redundant control of adenovirus late gene expression by early region 4

A series of human adenovirus type 5 derivatives carrying deletion mutations in early region 4 (E4) were constructed and characterized with respect to viral late protein synthesis, viral cytoplasmic late message accumulation, viral DNA accumulation, and plaquing ability. Viral late protein synthesis was essentially normal in cells infected by mutants expected to produce either the E4 open reading frame (ORF) 3 product or the E4 ORF 6 product. In cells infected by mutants lacking both ORF 3 and ORF 6, late protein synthesis was dramatically reduced. The basis for this reduction appears to be a concomitant reduction in cytoplasmic late message levels. Our results suggest that the products of ORFs 3 and 6 are redundant, since they are individually able to satisfy the requirement for E4 in late gene expression. Two of the mutants examined were defective for viral late protein synthesis but showed no measurable defect in viral DNA accumulation. The defect in late gene expression is not, therefore, a reflection of a primary defect in viral DNA synthesis. Finally, mutants expected to express ORF 3 or ORF 6 formed plaques with normal or only modestly reduced efficiency, whereas mutants expected to express neither ORF formed plaques with an efficiency less than 10(-6) that of wild-type virus. Thus, plaque-forming ability reflected late protein synthetic ability, suggesting that among these mutants late protein synthetic proficiency is the principle determinant of plaquing efficiency.

Characterization of adenovirus particles made by deletion mutants lacking the fiber gene

H2dl802, H2dl807, and H5dl1021 are defective deletion mutants of human adenovirus which do not make the capsid protein fiber yet which can make substantial amounts of virus particles. Virions made by the mutants contain very little fiber (which comes from helper virus contaminants in the deletion virus stocks): less than 6% as much as that contained by wild-type virions. This demonstrates that fiber is not an essential structural component of the adenovirus virion and suggests that fiber is nonessential for virion assembly. These fiber-deficient particles are poorly adsorbed to cells, consistent with the proposed role of fiber in virus attachment. Further, virion protein precursors, including that of the virion protease, are poorly processed in these particles, suggesting a relationship between the presence of fiber and the maturation of the virus particle.