An insertion mutation in the adenovirus type 12 early region 1A 13S mRNA unique region

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

Recombinant human adenoviruses containing hybrid adenovirus type 5 (Ad5)/Ad12 E1A genes: characterization of hybrid E1A proteins and analysis of transforming activity and host range

Hybrid adenovirus type 12 (Ad12)/Ad5 E1A genes were constructed by homologous recombination in Escherichia coli, a technique which offers several advantages over conventional mutagenesis for genetic analysis of proteins. In particular, functional differences between the proteins can be mapped by correlating the replacement of specific sequences with the acquisition of new properties, and there is no requirement for common unique restriction sites or polymerase chain reaction strategies to construct the hybrids. Recombinant adenoviruses expressing these hybrid E1A proteins were capable of replicating efficiently in HeLa cells, with the exception of one construct which contained a hybrid transactivation domain. The transforming activity of the hybrid E1A constructs was assayed by DNA transfection of primary baby rat kidney cells. Plasmids containing Ad12 E1 were approximately 20-fold less efficient at transformation than those with E1 of Ad5, and it was found that two regions in exon 1 of E1A mediate this difference. No differences were found in the abilities of any hybrid E1A proteins to bind to cellular proteins previously determined to be important for transformation by E1A.

Frequency of cell transformation by the small DNA tumor viruses: infection of proliferating cells and quiescent cells

Small DNA-containing tumor viruses (simian virus 40, mouse polyoma-virus, and adenoviruses) malignantly transform fibroblasts of the susceptible rodents. Fibroblasts can exist, in vitro and in vivo, in either of the two states: the proliferating state or the quiescent state. In the present study, we examined whether the state of fibroblasts at the time of exposure to these DNA viruses affects the frequency of transformation. Dense-focus formation in monolayer culture of rat 3Y1 fibroblasts was used to quantitate transformation. Results show that the frequency of transformation by simian virus 40 and mouse polyomavirus was reduced when cells were in the proliferating state at the time of virus inoculation as compared to cells in the quiescent state, whereas that by adenovirus type 12 was similar in the two cellular states. The reduction of the frequency of transformation in proliferating cells infected with simian virus 40 was also observed in BALB/c 3T3 mouse cells. Mechanisms underlying the difference between the two cellular states and the difference between the papovavirus and adenovirus in this aspect of transformation remain to be investigated.

The high sensitivity of cells transformed by E1A gene of adenovirus type 12 to diacylglycerol-mediated cell killing

Viability of rat 3Y1 fibroblasts transformed by adenovirus type 12 (Ad12) was markedly impaired by the administration of dilinoleoylglycerol (DLG) to the culture medium. To identify the gene(s) of Ad12 responsible for the high sensitivity to DLG, we established several transformed sublines of 3Y1 induced by the viral E1A gene or by the mutants of Ad12 which have mutations in the E1A region. All of the transformed sublines of 3Y1 expressing either the 12S or the 13S message from the E1A region were highly sensitive to the cytotoxicity of DLG. We propose that the high sensitivity of Ad12-transformed cells to the DLG mediated cytotoxicity is attributable to the common function of E1A-12S and E1A-13S mRNA products.

Suppression of block to entry into S phase in cell-cycle mutants of rat 3Y1 fibroblasts after transformation by adenovirus type 12

Four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts, belonging to separate complementation groups, cease to proliferate in the G1 phase of the cell cycle at a restrictive temperature (39.8 degrees). These ts mutants were transformed at a permissive temperature with adenovirus type 12 (Ad12), its E1A gene, or in203S mutant of Ad12 which has a mutation in the E1A 13 S mRNA unique region. We examined whether the proliferation of the transformed cells would be blocked in the G1 phase, at 39.8 degrees. One mutant did not cease to proliferate at 39.8 degrees after transformation with either Ad12 or E1A. In two other mutants, Ad12-transformed cells did not cease to proliferate at 39.8 degrees, whereas E1A-transformed cells did not survive at 39.8 degrees, though they did continue to enter the S phase. Analysis of transcription of the viral early genes in the transformants of one of the latter two mutants suggests that the expression of other viral early genes, in addition to E1A, is required for cell proliferation, in addition to entry into S phase. In the fourth mutant, both Ad12- and E1A-transformed sublines did not cease to enter the S phase but cells readily detached from the dishes. These results suggest that (1) function(s) of the E1A gene alone is sufficient for Ad12 to suppress the inhibition of the initiation of cellular DNA synthesis caused by four different cellular ts defects, (2) functions of Ad12 early genes other than, or in addition to, E1A are necessary for suppression of the inhibition of cell proliferation (and/or for survival) in two of the four ts mutants, and (3) in the case of one other ts mutant, E1A alone overcomes the ts defect and allows for the entire cell proliferation.

Host range mutants of adenovirus type 12 E1 defective for lytic infection, transformation, and oncogenicity

The human adenovirus type 12 (H12) E1A region encodes two early proteins of 266 amino acid residues (266R) and 235R whilst the H12 E1B promoter directs the synthesis of two major proteins of 163R and 482R. To determine the functions of E1A and E1B in lytic infection and oncogenic transformation we have isolated and characterized a series of H12 E1 mutants. Mutant H12 hr 700 contains a point mutation in exon 1 that alters a single amino acid common to both the 266 and 235R proteins. This mutant synthesized reduced levels of E1 and structural proteins at delayed times in HEK cells, transformed BRK cells, and induced tumors in newborn rats at reduced efficiency compared to wild-type virus. The mutation in H12 in 600 truncates the 266R protein in its unique sequences but this mutant synthesized the 235R, E1B, and structural proteins at delayed times in HEK cells. H12 in 600 was nontransforming but induced rare tumors in newborn rats. A third E1A mutant H12 in 601 synthesized no E1A proteins, reduced levels of E1B and structural proteins at delayed times in lytic infections, and was not a transforming or oncogenic virus. Three E1B mutants were studied in detail. Both H12 hr 703 and H12 in 602 encode N-terminal truncated 482R proteins whereas H12 del 620 encodes an in-frame internally deleted 482R protein. All three synthesized reduced amounts of E1A proteins and the E1B 163R protein, identifying a regulatory function for the 482R protein. None of the E1B mutants could transform and only H12 del 620 could induce rare tumors in newborn rats. These results show that H12 oncogenesis requires the coordinated expression of the E1 proteins.

Activation of the human beta interferon gene by the adenovirus type 12 E1B gene

The transcription of endogenous beta interferon mRNA was activated in human embryo kidney (HEK) cells infected with adenovirus type 12 (Ad12) but was activated only inefficiently or not at all in HEK cells infected with Ad5 and rc-1 (Ad5 dl312 containing the Ad12 E1A region). The analysis with Ad12 mutants showed that Ad12 E1B products, especially the 19K protein, were important for the expression of the endogenous beta interferon gene and Ad12 E1A products were not involved in the expression. The expression of exogenously transfected pIFN-CAT (a hybrid plasmid having the human beta interferon promoter fused with the CAT gene) was activated in HEK and chicken embryo fibroblast (CEF) cells infected with either Ad12 or Ad5. The analysis of cotransfection of CEF cells with pIFN-CAT and plasmids containing fragments of Ad12 or Ad5 DNA showed that Ad12 or Ad5 E1B (possibly the 19K protein) was and E1A was not involved in the expression of the exogenous pIFN-CAT.

Induction of cellular DNA synthesis by adenovirus type 12 in a set of temperature-sensitive mutants of rat 3Y1 fibroblasts blocked in G1 phase

Four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts, which represent separate complementation groups, cease to proliferate predominantly with a 2C DNA content, either at 39.8 degrees (temperature arrest), or at 33.8 degrees at a confluent cell density (density arrest). When infected at 39.8 degrees with adenovirus type 12 (Ad12), cells of all four ts mutants in both arrest states entered the S phase, thereby suggesting that Ad12 overcomes the four independent functional blocks to cellular entry into S phase. Results of experiments using Ad12 E1-region mutants suggest that the E1A gene product(s) is indispensable to overcoming the ts block, whereas the E1B product(s) may be dispensable. The cell killing observed in 3Y1 cells infected with wild-type Ad12 did not occur in infection with one of the E1-region mutants with a 6-bp insertion in the E1A 13 S mRNA unique region. When infected with this mutant at 39.8 degrees, two ts mutants of 3Y1 (3Y1tsF121 and 3Y1tsG125) in both arrested states proliferated through at least one generation. Another mutant (3Y1tsD123) was accelerated to die following entry into the S phase. In the other mutant (3Y1tsH203), the cell number was either unchanged (temperature arrest) or was increased less than twofold and then decreased (density arrest). The findings with the latter two mutant lines suggest that induction of cellular DNA synthesis is not sufficient for the subsequent proliferation of the infected cells, and that the Ad12 gene function(s) does not directly rescue the primary lesions in these ts mutants but does overcome some of the blocks to concomitantly occurring events. In the former two mutant lines, however, Ad12 gene function(s) may directly rescue the ts lesions. We propose that the Ad12 gene product(s) can overcome blocks to the initiation of cellular DNA synthesis but cannot overcome blocks to events related to cell survival.

Interferon induction by adenovirus type 12: stimulatory function of early region 1A

Adenoviruses are generally weak interferon inducers, triggering chicken embryo fibroblast cells by a UV-resistant viral component, probably the capsid or capsid elements, to produce 50 to 100 IU of interferon per ml. Adenovirus types 12, 18, and 31, however, can induce by a UV-sensitive mechanism 10 to 20 times more interferon than other types do. By using mutant and recombinant adenoviruses, we demonstrated that early region 1A was responsible for the enhanced interferon production of chicken cells infected with adenovirus type 12.

The adenovirus type 12 early-region 1B 58,000-Mr gene product is required for viral DNA synthesis and for initiation of cell transformation

An E1B 58K mutant of adenovirus type 12 (Ad12), dl207, was constructed by the deletion of 852 base pairs in the E1B 58K coding region. The mutant could grow efficiently in 293E1 cells but not in HeLa, KB, or human embryo kidney (HEK) cells. Viral DNA replication of dl207 was not detected in HeLa and KB cells and was seldom detected in HEK cells. Analysis of viral DNA synthesis in vitro showed that the Ad12-DNA-protein complex replicated by using the nuclear extract from Ad12 wild-type (WT)-infected HeLa cells but not by using the nuclear extract from dl207-infected cells. In dl207-infected HeLa and KB cells, early mRNAs were detected, but late mRNAs were not detected. The mutant induced fewer transformed foci than the WT in rat 3Y1 cells. Cells transformed by dl207 could grow efficiently in fluid medium, form colonies in soft agar culture, and induce tumors in rats transplanted with the transformed cells at the same efficiency as WT-transformed cells. Tumors were induced in hamsters injected with WT virions but were not induced in hamsters injected with dl207 virions. The results indicate that the E1B 58K protein is required both for viral DNA replication in productive infection and for initiation of cell transformation, but not for maintenance of the transformed phenotype.