Isolation and a preliminary characterization of temperature-sensitive mutants of adenovirus 12

The adenovirus DNA-binding protein DBP

Adenoviruses are a group of double-stranded DNA viruses that can mainly cause respiratory, gastrointestinal, and eye infections in humans. In addition, adenoviruses are employed as vector vaccines for combatting viral infections, including SARS-CoV-2, and serve as excellent gene therapy vectors. These viruses have the ability to modulate the host cell machinery to their advantage and trigger significant restructuring of the nuclei of infected cells through the activity of viral proteins. One of those, the adenovirus DNA-binding protein (DBP), is a multifunctional non-structural protein that is integral to the reorganization processes. DBP is encoded in the E2A transcriptional unit and is highly abundant in infected cells. Its activity is unequivocally linked to the formation, structure, and integrity of virus-induced replication compartments, molecular hubs for the regulation of viral processes, and control of the infected cell. DBP also plays key roles in viral DNA replication, transcription, viral gene expression, and even host range specificity. Notably, post-translational modifications of DBP, such as SUMOylation and extensive phosphorylation, regulate its biological functions. DBP was first investigated in the 1970s, pioneering research on viral DNA-binding proteins. In this literature review, we provide an overview of DBP and specifically summarize key findings related to its complex structure, diverse functions, and significant role in the context of viral replication. Finally, we address novel insights and perspectives for future research.

Isolation and characterization of temperature-sensitive mutants of adenovirus type 7

Fifty temperature-sensitive mutants, which replicate at 32 degrees C but not at 39.5 degrees C, were isolated after mutagenesis of the vaccine strain of adenovirus type 7 with hydroxylamine (mutation frequency of 9.0%) or nitrous acid (mutation frequency of 3.8%). Intratypic complementation analyses separated 46 of these mutants into seven groups. Intertypic complementation tests with temperature-sensitive mutants of adenovirus type 5 showed that the mutant in complementation group A failed to complement H5ts125 (a DNA-binding protein mutant), that mutants in group B and C did not complement adenovirus type 5 hexon mutants, and that none of the mutants was defective in fiber production. Further phenotypic characterization showed that at the nonpermissive temperature the mutant in group A failed to make immunologically reactive DNA-binding protein, mutants in groups B and C were defective in transport of trimeric hexons to the nucleus, mutants in groups D, E, and F assembled empty capsids, and mutants in group G assembled DNA-containing capsids as well as empty capsids. The mutants of the complementation groups were physically mapped by marker rescue, and the mutations were localized between the following map coordinates: groups B and C between 50.4 and 60.2 map units (m.u.), groups D and E between 29.6 and 36.7 m.u., and group G between 36.7 and 42.0 m.u. or 44.0 and 47.0 m.u. The mutant in group A proved to be a double mutant.

Phenotypic characterization of adenovirus type 12 temperature-sensitive mutants in productive infection and transformation

Eleven temperature-sensitive mutants of adenovirus type 12, capable of forming plaques in human cells at 33 C but not at 39.5 C, were isolated from a stock of a wild-type strain after treatment with either nitrous acid or hydroxylamine. Complementation tests in doubly infected human cells permitted a tentative assignment of eight of these mutants to six complementation groups. Temperature-shift experiments revealed that one mutant is affected early and most of the other mutants are affected late. Only the early mutant, H12ts505, was temperature sensitive in viral DNA replication. Infectious virions of all the mutants except H12ts505 and two of the late mutants produced at 33 C, appeared to be more heat labile than those of the wild type. Only H12ts505 was temperature sensitive for the establishment of transformation of rat 3Y1 cells. One of the late mutants (H12ts504) had an increased transforming ability at the permissive temperature. Results of temperature-shift transformation experiments suggest that a viral function affected in H12ts505 is required for "initiation" of transformation. Some of the growth properties of H12ts505-transformed cells were also temperature dependent, suggesting that a functional expression of a gene mutation in H12ts505 is required to maintain at least some aspects of the transformed state.

Morphogenesis of human adenovirus type 2 studied with fiber- and fiber and penton base-defective temperature-sensitive mutants

The nature, polypeptide composition, and antigenic composition of the particles formed by six human adenovirus type 2 temperature-sensitive (ts) mutants were studied. ts115, ts116, and ts125 were phenotypically fiber-defective mutants, and ts103, ts104, and ts136 failed to synthesize detectable amounts of fiber plus penton base at 39.5 degrees C. The mutants belonged to five complementation groups, one group including ts116 and ts125. Except for ts103 and ts136, the other mutants were capable of producing particles at 39.5 degrees C. ts116 and ts125 accumulated light assembly intermediate particles (or top components) at nonpermissive temperatures, with few virus particles. The sodium dodecyl sulfate polypeptide pattern of ts116- or ts125-infected cells, intermediate particles, and virus particles showed that polypeptide IV (fiber) was smaller by a molecular weight of 2,000 than that in the wild-type virion and was glycosylated. In fiber plus penton base-defective ts104-infected cells, equivalent quantities of top components and viruses with a buoyant density (rho) of 1.345 g/ml (rho = 1.345 particles) were produced at 39.5 degrees C. These rho = 1.345 particles corresponded to young virions, as evidenced by the presence of uncleaved precursors to proteins VI, VIII, and VII. These young virions matured upon a shift down. Virus capsid vertex antigenic components underwent a phase of eclipse during their incorporation into mature virus particles. No antigenic penton base or IIa was detected in intermediate particles of all the ts mutants tested. Only hexon and traces of fiber antigens were found in ts104 young virions. Penton base and IIIa appeared as fully antigenically expressed capsid subunits in mature wild-type virions or ts104 virions after a shift down. The ts104 lesion is postulated to affect a regulatory function related in some way to penton base and fiber overproduction and the maturation processing of precursors PVI, PVII, and PVII.

Characterization of a temperature-sensitive mutant of human adenovirus type 7

The properties of a naturally occurring temperature-sensitive (ts) mutant of human adenovirus type 7 (Ad7) were studied. Mutant Ad7 (19), or E46-, was the nonhybrid adenovirus component derived from the defective simian virus 40 (SV40)-Ad7 hybrid (PARA). Growth of the mutant was restricted at 40.5 degrees C, and the ratios of virus yields in KB cells at 40.5 and 33 degrees C were 10(-2) to 10(-3). Viral DNA synthesis and the synthesis of adenovirus-specific antigens (tumor, capsid, hexon, and penton antigens) appeared normal at the restrictive temperature. The assembly of virus particles was aberrant, as determined by thin-section of infected cells. The infectivity of mutant virions was heat labile at 50 degrees C, suggesting a ts defect in a structural component of the viron. Analysis by polyacrylamide gel electrophoresis of [35S]methionine-labeled polypeptides synthesized in mutant-infected cells suggested that at least the major virion polypeptides were synthesized at the restrictive temperature. A lack of inhibition of host protein synthesis late in mutant infections, as compared with wild-type (WT) infections at both the permissive and nonpermissive temperatures, made quantitation of infected-cell polypeptides difficult. Analysis of the assembly of capsomeres from cytoplasmic extracts of infected cells on sucrose gradients and by non-dissociating polyacrylamide gel electrophoresis suggested that hexon capsomeres were made at 40.5 degrees C. The hexon capsomeres made by the mutant at either 33 or 40.5 degrees C displayed a decreased migration in the non-dissociating gels compared with the WT hexon capsomeres. The molecular weights of the mutant and WT hexon polypeptides were identical. These results suggest that the ts lesion of this group B human Ad7 mutant may be reflected in altered hexons. The mutant Ad7 interfered with the replication of adenovirus types 2 and 21 at the elevated temperature.

Viral DNA synthesis in vitro with the inclusions isolated from adenovirus 12-infected cells

A fraction defined as the inclusions was isolated by banding in CsC1 gradients from nuclei of adenovirus 12-infected KB cells. When examined by electron microscopy, the isolated inclusions were relatively homogeneous, finely granular materials of moderate electron density, possibly representing the disintegrated type II or IV inclusions. The conditions of endogenous DNA synthesis in vitro with the inclusions were determined. The product of DNA synthesis in vitro with the inclusions was mainly viral and scarcely cellular, as revealed by DNA-DNA hybridization and methylated albumin kieselgur column chromatography. However, viral DNA synthesized in vitro was smaller (18S, 22S) than viral DNA in virions (31 S, 34 S) in neutral and alkaline sucrose gradients. Effects of various treatment of the inclusions on the DNA-synthesizing activity showed that phospholipase C inhibited the activity efficiently. The in vitro DNA synthesis was stimulated by addition of the cytoplasmic extract from adenovirus 12-infected cells and not that from unifected cells. The analysis of the composition of the inclusions showed that the inclusions contained DNA, protein, phospholipid and a small amount of RNA and carbohydrate.

Characterization of a temperature-sensitive, hexon transport mutant of type 5 adenovirus

Infection of KB cells at 39.5 degrees C with H5ts147, a temperature-sensitive (ts) mutant of type 5 adenovirus, resulted in the cytoplasmic accumulation of hexon antigen; all other virion proteins measured, however, were normally transported into the nucleus. Immunofluorescence techniques were used to study the intracellular location of viral proteins. Genetic studies revealed that H5ts147 was the single member of a nonoverlapping complementation group and occupied a unique locus on the adenovirus genetic map, distinct from mutants that failed to produce immunologically reactive hexons at 39.5 degrees C ("hexon-minus" mutants). Sedimentation studies of extracts of H5ts147-infected cells cultured and labeled at 39.5 degrees C revealed the production of 12S hexon capsomers (the native, trimeric structures), which were immunoprecipitable to the same extent as hexons synthesized in wild type (WT)-infected cells. In contrast, only 3.4S polypeptide chains were found in extracts of cells infected with the class of mutants unable to produce immunologically reactive hexon protein at 39.5 degrees C. Hexons synthesized in H5ts147-infected cells at 39.5 degrees C were capable of being assembled into virions, to the same extent as hexons synthesized in WT-infected cells, when the temperature was shifted down to the permissive temperature, 32 degrees C. Infectious virus production was initiated within 2 to 6 h after shift-down to 32 degrees C; de novo protein synthesis was required to allow this increase in viral titer. If ts147-infected cells were shifted up to 39.5 degrees C late in the viral multiplication cycle, viral production was arrested within 1 to 2 h. The kinetics of shutoff was similar to that of a WT-infected culture treated with cycloheximide at the time of shift-up. The P-VI nonvirion polypeptide, the precursor to virion protein VI, was unstable at 39.5 degrees C, whereas the hexon polypeptide was not degraded during the chase. It appears that there is a structural requirement for the transport of hexons into the nucleus more stringent than the acquisition of immunological reactivity and folding into the 12S form.

Isolation and characterization of temperature-sensitive mutants of adenovirus type2

Fourteen temperature-sensitive mutants of human adenovirus type2, which differed in their plaquing efficiencies at at the permissive and nonpermissive temperatures by 4 to 5 orders of magnitude, were isolated. These mutants, which could be assigned to seven complementation groups, were tested for their capacity to synthesize adenovirus DNA at the nonpermissive temperature. Three mutants in three different complementation groups proved deficient in viral DNA synthesis. The DNA-negative mutant H2ts206 complemented the DNA-negative mutants H5ts36 and H5ts125, whereas mutant H2ts201 complemented H5ts36 only. Among the DNA-negative mutants, H2ts206 synthesized the smallest amount of viral DNA at the nonpermissive temperature (39.5 C). Data obtained in temperature shift experiments indicated that a very early function was involved in temperature sensitivity. In keeping with this observation, early virus-specific mRNA was not detected in cells infected with H2ts206 and maintained at 39.5 C. Prolonged (52 h) incubation of cells infected with H2ts206 at the nonpermissive temperature led to the synthesis of a high-molecular-weight form of viral DNA.

Characterization of an adenovirus early protein required for viral DNA replication: a single strand specific DNA binding proteins

1. The human adenoviruses types 2, 5 and 12 code for the production of a single strand specific DNA binding protein. The molecular weights of these proteins were 72,000 for types 2 and 5 and 60,000 for type 12. In all three cases proteolytic breakdown fragments of these binding proteins (48,000 MW) were also observed. 2. Analysis of the methionine containing tryptic peptides of these proteins indicate that the types 2 and 5 proteins are similar and clearly distinguishable from the type 12 protein. The peptide maps of these three viral proteins are clearly different from a similar protein found in mock infected cells. 3. Temperature sensitive mutants of type 5 (H5ts125) and type 12(H12tsA275) adenoviruses fail to produce these proteins at the nonpermissive temperature. H5ts125 infected cells grown at the permissive temperature produce a 72,000 MW protein that is thermolabile, for continued binding to DNA, when compared to type 5 wild type adenovirus 72,000 MW protein. An analysis of the phenotype of this adenovirus mutant indicates that it codes for a viral function at early times after infection that is required for viral DNA replication. 4. The in vitro translation of adenovirus specific m-RNA results in the synthesis of a small amount of a 72,000 MW protein that binds to single stranded DNA just like the authentic adenovirus DNA binding proteins produced in infected cells. 5. Adenovirus anti-Tumor antigen (T) anti-serum from hamsters carrying independently derived adenovirus tumors, have been tested for the presence of antibody to purified DNA binding proteins. One antiserum is positive for these antibodies while the other is negative. These results indicate that some, but not all, adenovirus tumors contain large enough levels of the DNA binding proteins to elicit an antibody response. 6. The type 5 adenovirus temperature sensitive mutant, H5ts125, that codes for a thermolabile DNA binding protein, was complemented or suppressed at the nonpermissive temperature, for the replication of adenovirus DNA, by SV40. SV40tsA temperature sensitive mutants, defective in SV40 DNA replication, do not suppress or complement H5ts125 at the nonpermissive temperature.

The location of the genes coding for hexon and fiber proteins in adenovirus DNA

A serological analysis has been made of the capsid antigens hexon and fiber from 17 Ad5-Ad2+ND1 recombinants that enables us to determine the phenotype of the recombinants. By correlation of this data with the genetic and physical maps of the adenovirus genome, obtained by recombination and restriction endonuclease analysis, the genes coding for the hexon and fiber have been assigned to specific locations on the adenovirus DNA.

Genetic analysis of adenovirus type 2. I. Isolation and genetic characterization of temperature-sensitive mutants

Temperature-sensitive mutants which replicate normally at 33 C but poorly at 39 C were isolated from nitrosoguanidine- or nitrous acid-mutagenized adenovirus 2 by (i) testing the cytopathic effect or inclusion body-forming capacity of random plaque isolates, or (ii) reduced plaque enlargement upon shifting from 33 to 39 C. Thirty-six mutants were isolated with 33 C/39 C plaque ratios varying from 20 to 10-5. Some of these mutants could be arranged into 13 groups by the complementation test. By means of recombination analysis a provisional linear genetic map was constructed.

Temperature-sensitive mutants of adenovirus type 12 defective in viral DNA synthesis

Ten temperature-sensitive (ts) mutants of adenovirus type 12 which produce plaques at 31 but not at 38.5 C have been isolated after mutagenesis with nitrosoguanidine or nitrous acid. The mutants have been classified into six separate complementation groups. DNA-DNA hybridizations have shown that at 38.5 C the ts 401 and 406 mutants of groups B and E, respectively, synthesized less than 10% of the normal level of viral DNA. The two mutants were also defective in the production of late proteins at the nonpermissive temperature, as shown by fluorescent-antibody tests and analysis by sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Genetic recombination between the ts viruses 401 and 406 has been demonstrated; the recombination frequency for the wild-type virus production was 17.7%. Both mutants induced an increase in thymidine kinase activity at 38.5 C. Moreover, the two viral DNA-defective mutants shut off host DNA synthesis at the restrictive temperature. It is striking that at 38.5 C ts virus 401 transformed two to eight times more hamster cells than the wild-type virus, whereas ts virus 406 transformed at a frequency similar to the wild-type virus.

Proposed nomenclature for mutants of adenoviruses

In accord with the nomenclature proposed for mutants of simian virus 40 the same rules, with minor modifications, are recommended for naming mutants of adenoviruses. It is further suggested that these rules, which pertain to a system of classification based primarily upon complementation analysis, also be applied to mutants of other DNA-containing animal viruses.

Nonspecific adsorption of proteins to microplates

When a solution of purified adenovirus hexon proteins was diluted in polystyrene microplates without carrier proteins in the diluent, a higher rate of hexon adsorption to the microplates at higher dilutions took place. Addition of gelatin (0.01%), bovine serum albumin (0.01%), or calf serum (0.2%, vol/vol) to the diluent effectively prevented this adsorption. The adsorption to the microplates of adenovirions and measles virions was also prevented by the carrier proteins in the diluent. Certain additional features which distinguish the microtechnique of Takatsy from conventional macromethods are also discussed.