Evidence for unique DNA repair activity encoded by a cloned Serratia marcescens gene: suppression of Escherichia coli mutations that reduce repair of alkylated DNA.

A recombinant plasmid containing a Serratia marcescens DNA repair gene has been analyzed biochemically and genetically in Escherichia coli mutants deficient for repair of alkylated DNA. The cloned gene suppressed sensitivity to methyl methanesulfonate of an E. coli strain deficient in 3-methyladenine DNA glycosylases I and II (i.e., E. coli tag alkA) and two different E. coli recA mutants. Attempts to suppress the methyl methanesulfonate sensitivity of the E. coli recA mutant by using the cloned E. coli tag and alkA genes were not successful. Southern blot analysis did not reveal any homology between the S. marcescens gene and various known E. coli DNA repair genes. Biochemical analysis with the S. marcescens gene showed that the encoded DNA repair protein liberated 3-methyladenine from alkylated DNA, indicating that the DNA repair molecular is an S. marcescens 3-methyladenine DNA glycosylase. The ability to suppress both types of E. coli DNA repair mutations, however, suggests that the S. marcescens gene is a unique bacterial DNA repair gene.     

Influenza virus hemagglutinin (H3 subtype) requires palmitoylation of its cytoplasmic tail for assembly: M1 proteins of two subtypes differ in their ability to support assembly

The influenza A virus hemagglutinin (HA) transmembrane domain boundary region and the cytoplasmic tail contain three cysteines (residues 555, 562, and 565 for the H3 HA subtype) that are highly conserved among the 16 HA subtypes and which are each modified by the covalent addition of palmitic acid. Previous analysis of the role of these conserved cysteine residues led to differing data, suggesting either no role for HA palmitoylation or an important role for HA palmitoylation. To reexamine the role of these residues in the influenza virus life cycle, a series of cysteine-to-serine mutations were introduced into the HA gene of influenza virus A/Udorn/72 (Ud) (H3N2) by using a highly efficient reverse genetics system. Mutant viruses containing HA-C562S and HA-C565S mutations had reduced growth and failed to form plaques in MDCK cells but formed wild-type-like plaques in an MDCK cell line expressing wild-type HA. In cell-cell fusion assays, nonpalmitoylated H3 HA, in both cDNA-transfected and virus-infected cells, was fully competent for HA-mediated membrane fusion. When the HA cytoplasmic tail cysteine mutants were examined for lipid raft association, using as the criterion Triton X-100 insolubility, loss of raft association did not show a direct correlation with a reduction in virus replication. However, mutant virus assembly was reduced in parallel with reduced virus replication. Additionally, a reassortant of strain A/WSN/33 (WSN), containing the Ud HA gene with mutations C555S, C562S, and C565S, produced virus that could form plaques on regular MDCK cells and had only moderately decreased replication, suggesting differences in the interactions between Ud and WSN HA and internal viral proteins. Analysis of M1 mutants containing substitutions in the six residues that differ between the Ud and WSN M1 proteins indicated that a constellation of residues are responsible for the difference between the M1 proteins in their ability to support virus assembly with nonpalmitoylated H3 HA.     

Synthetic lethality of Drosophila in the absence of the MUS81 endonuclease and the DmBlm helicase is associated with elevated apoptosis

Mus81-Mms4 (Mus81-Eme1 in some species) is a heterodimeric DNA structure-specific endonuclease that has been implicated in meiotic recombination and processing of damaged replication forks in fungi. We generated and characterized mutations in Drosophila melanogaster mus81 and mms4. Unlike the case in fungi, we did not find any role for MUS81-MMS4 in meiotic crossing over. A possible role for this endonuclease in repairing double-strand breaks that arise during DNA replication is suggested by the finding that mus81 and mms4 mutants are hypersensitive to camptothecin; however, these mutants are not hypersensitive to other agents that generate lesions that slow or block DNA replication. In fungi, mus81, mms4, and eme1 mutations are synthetically lethal with mutations in genes encoding RecQ helicase homologs. Similarly, we found that mutations in Drosophila mus81 and mms4 are synthetically lethal with null mutations in mus309, which encodes the ortholog of the Bloom Syndrome helicase. Synthetic lethality is associated with high levels of apoptosis in proliferating tissues. Lethality and elevated apoptosis were partially suppressed by a mutation in spn-A, which encodes the ortholog of the strand invasion protein Rad51. These findings provide insights into the causes of synthetic lethality.     

A cis-acting sequence promotes removal of simian virus 40 DNA from the replication pool.

Pulse-labeled simian virus 40 (SV40) DNA is removed from the pool of molecules available for replication (i.e., it ceases to reenter replication) a few hours after synthesis. We studied this cessation of reentry with mutants containing different deletions in the structural genes of SV40. The DNAs of two independent deletion mutants, dl-1007 (24% deletion) and dl-1003 (8% deletion), were used as templates for further DNA synthesis (i.e., they reentered replication) to a greater extent than was wild-type DNA. The alteration in reentry kinetics was not because the DNAs were smaller; other deletion mutations that were from 76 to 85% of the length of wild-type DNA (dl-BE and dl-1133 with a deletion in the late region and F8dl with a deletion in the early region) did not reenter replication to a greater extent than the wild type did. Cotransfection experiments showed that the mutant phenotypes of dl-1007 and dl-1003 were poorly complemented, if at all, by the wild type. Thus, we propose that there is a cis-acting sequence located in the HindIII E fragment of SV40, not present in either of these mutants, that promotes the efficient removal of DNA from the replication pathway.     

Mutagenic effect of nitrosomethylurea, modified by heat shock at early stages of sunflower seedling development]

A comparative study of chlorophyll mutations induced in sunflower seedlings of the initial line 3629 and its nuclear and plastom mutant derivatives by nitrosomethylurea (NMU) (0.015%), heat shock (HS, 40 degrees C), and their combination was performed. The spectrum and frequency of chlorophyll mutations depended on the treatment conditions and on the genotype of the lines tested. Nuclear mutant displayed highest sensitivity to NMU, HS, and their combination. The line 3629 plants displayed highest tolerance to NMU and HS. However, plastom mutants were more tolerant to the combination of the mutagens compared to the plants of the initial line.     

Mutagenic Effect of Nitrosomethylurea Modified by Heat Shock at Early Stages of the Sunflower Seedlings Development

A comparative study of chlorophyll mutations induced in sunflower seedlings of the initial line 3629 and its nuclear and plastom mutant derivatives by nitrosomethylurea (NMU), heat shock (HS, 40°C), and their combination was performed. The spectrum and frequency of chlorophyll mutations depended on the treatment conditions and on the genotype of the lines tested. Nuclear mutant displayed highest sensitivity to NMU, HS, and their combination. The line 3629 plants displayed highest tolerance to NMU and HS. However, plastom mutants were more tolerant to the combination of the mutagens compared to the plants of the initial line.     

Simian virus 40 mutants carrying two temperature-sensitive mutations.

Five temperature-sensitive mutants of simian virus 40 containing two temperature-sensitive mutations were isolated. The double mutant of the A and D complementation groups, like the D mutants, failed to complement by conventional complementation analysis and did not induce host DNA synthesis at 40 degrees C. However, under conditions that suppressed the D defect, the A:D double mutant expressed only the A defect. Thus, viral DNA replication dropped rapidly after this mutant was shifted from permissive to restrictive temperatures. The A:D double mutant failed to transfrom at the restrictive temperature when subconfluent Chinese hamster lung monolayers were used. Double mutants of A:B, A:C, and A:BC complementation groups, like their A parent, were defective in viral DNA replication, in the induction of host DNA synthesis and in the transformation of secondary Chinese hamster lung cells at the nonpermissive temperature.     

Induction of intracellular endonuclease synthesis in Serratia marcescens by agents suppressing DNA replication]

Endonuclease synthesis in Serratia marcescens was studied in the presence of agents selectively suppressing DNA biosynthesis: nalidixic acid, mitomycin, hydroxyurea and thymine limitation. All the agents suppressing DNA replication induced exocellular endonuclease biosynthesis irrespective of their action mechanism. The greatest inducing effect was exerted when the agents were added to cells in the late exponential phase. Endonuclease biosynthesis was induced 1-2 hours after adding the agent and was inhibited with chloramphenicol. The induction of exocellular endonuclease synthesis in Serratia marcescens by the classical inducing agents of a SOS response seems to be indicative of a Lex A regulated process.     

Simian virus 40-transformed human cells that express large T antigens defective for viral DNA replication.

Many types of human cells cultured in vitro are generally semipermissive for simian virus 40 (SV40) replication. Consequently, subpopulations of stably transformed human cells often carry free viral DNA, which is presumed to arise via spontaneous excision from an integrated DNA template. Stably transformed human cell lines that do not have detectable free DNA are therefore likely to harbor harbor mutant viral genomes incapable of excision and replication, or these cells may synthesize variant cellular proteins necessary for viral replication. We examined four such cell lines and conclude that for the three lines SV80, GM638, and GM639, the cells did indeed harbor spontaneous T-antigen mutants. For the SV80 line, marker rescue (determined by a plaque assay) and DNA sequence analysis of cloned DNA showed that a single point mutation converting serine 147 to asparagine was the cause of the mutation. Similarly, a point mutation converting leucine 457 to methionine for the GM638 mutant T allele was found. Moreover, the SV80 line maintained its permissivity for SV40 DNA replication but did not complement the SV40 tsA209 mutant at its nonpermissive temperature. The cloned SV80 T-antigen allele, though replication incompetent, maintained its ability to transform rodent cells at wild-type efficiencies. A compilation of spontaneously occurring SV40 mutations which cannot replicate but can transform shows that these mutations tend to cluster in two regions of the T-antigen gene, one ascribed to the site-specific DNA-binding ability of the protein, and the other to the ATPase activity which is linked to its helicase activity.     

Cloning of the Serratia marcescens recA gene and construction of a Serratia marcescens recA mutant

A recombinant plasmid, pSM2513, containing an 8.5 kb DNA insert was isolated from a genomic library of Serratia marcescens by using interspecific complementation. This plasmid conferred resistance to methyl methanesulphonate and UV irradiation upon recA mutants of Escherichia coli and enhanced recombination proficiency, as measured by Hfr-mediated conjugation, in recA mutants of E. coli. Furthermore, when recA mutants of E. coli harbouring pSM2513 were subjected to UV irradiation, filamentation of the cells was observed. This did not occur upon UV irradiation of the same mutants harbouring the cloning vector alone. These results imply that the S. marcescens recA gene on pSM2513 is functionally similar to the E. coli recA gene in several respects. Restriction enzyme analysis and subcloning studies revealed that the S. marcescens recA gene was located on a 2.7 kb Bg/II-KpnI fragment of pSM2513, and its gene product of approximately 39 kDa resembled the E. coli RecA protein in molecular mass. Using transformation-mediated marker rescue, a recA mutant of S. marcescens was successfully constructed; its proficiency both in homologous recombination and in DNA repair was abolished compared with its parent.     

Amino acids in the basic domain of Epstein-Barr virus ZEBRA protein play distinct roles in DNA binding, activation of early lytic gene expression, and promotion of viral DNA replication

The ZEBRA protein of Epstein-Barr virus (EBV) drives the viral lytic cycle cascade. The capacity of ZEBRA to recognize specific DNA sequences resides in amino acids 178 to 194, a region in which 9 of 17 residues are either lysine or arginine. To define the basic domain residues essential for activity, a series of 46 single-amino-acid-substitution mutants were examined for their ability to bind ZIIIB DNA, a high-affinity ZEBRA binding site, and for their capacity to activate early and late EBV lytic cycle gene expression. DNA binding was obligatory for the protein to activate the lytic cascade. Nineteen mutants that failed to bind DNA were unable to disrupt latency. A single acidic replacement of a basic amino acid destroyed DNA binding and the biologic activity of the protein. Four mutants that bound weakly to DNA were defective at stimulating the expression of Rta, the essential first target of ZEBRA in lytic cycle activation. Four amino acids, R183, A185, C189, and R190, are likely to contact ZIIIB DNA specifically, since alanine or valine substitutions at these positions drastically weakened or eliminated DNA binding. Twenty-three mutants were proficient in binding to ZIIIB DNA. Some DNA binding-proficient mutants were refractory to supershift by BZ-1 monoclonal antibody (epitope amino acids 214 to 230), likely as the result of the increased solubility of the mutants. Mutants competent to bind DNA could be separated into four functional groups: the wild-type group (eight mutants), a group defective at activating Rta (five mutants, all with mutations at the S186 site), a group defective at activating EA-D (three mutants with the R179A, S186T, and K192A mutations), and a group specifically defective at activating late gene expression (seven mutants). Three late mutants, with a Y180A, Y180E, or K188A mutation, were defective at stimulating EBV DNA replication. This catalogue of point mutants reveals that basic domain amino acids play distinct functions in binding to DNA, in activating Rta, in stimulating early lytic gene expression, and in promoting viral DNA replication and viral late gene expression. These results are discussed in relationship to the recently solved crystal structure of ZEBRA bound to an AP-1 site.     

Hemagglutinin mutations related to attenuation and altered cell tropism of a virulent avian influenza A virus.

The H5 hemagglutinin (HA) of a highly virulent avian influenza virus, A/Turkey Ontario/7732/66 (H5N9), was previously shown to have five neutralizing epitopes, and escape mutants within one epitope (group 1) were markedly attenuated (M. Philpott, B. C. Easterday, and V. S. Hinshaw, J. Virol. 63:3453-3458, 1989). To define the genetic changes related to these antigenic and biologic properties, the HA genes of mutants within each of the epitope groups were sequenced by using the polymerase chain reaction. The mutations in the attenuated group 1 mutants were located near the distal tip of the HA molecule in close proximity to the receptor-binding site, on the basis of alignment with the three-dimensional structure of the H3 HA. All group 1 mutations involved charged amino acids. The group 1 mutants, similar to the wild-type virus, spread systemically and were recovered from the spleens of infected chickens but, unlike the wild-type virus, failed to produce severe necrosis in the spleens. Viral replication in the spleens was investigated by in situ hybridization of spleen sections from chickens infected with the wild-type or attenuated mutants. Wild-type virus replication was demonstrated in large, mononuclear, macrophagelike cells; however, group 1 mutant virus was detected attached only to erythrocytes within the red pulp. These results suggest that the attenuated mutants differ in their cell tropism within the spleen.     

Replication and mutagenesis of UV-damaged DNA templates in human and monkey cell extracts.

We have used in vitro DNA replication systems from human HeLa cells and monkey CV-1 cells to replicate a UV-damaged simian virus 40-based shuttle vector plasmid, pZ189. We found that replication of the plasmid was inhibited in a UV fluence-dependent manner, but even at UV fluences which caused damage to essentially all of the plasmid molecules some molecules became completely replicated. This replication was accompanied by an increase (up to 15-fold) in the frequency of mutations detected in the supF gene of the plasmid. These mutations were predominantly G:C-->A:T transitions similar to those observed in vivo. Treatment of the UV-irradiated plasmid DNA with Escherichia coli photolyase to reverse pyrimidine cyclobutane dimers (the predominant UV-induced photoproduct) before replication prevented the UV-induced inhibition of replication and reduced the frequency of mutations in supF to background levels. Therefore, the presence of pyrimidine cyclobutane dimers in the plasmid template appears to be responsible for both inhibition of replication and mutation induction. Further analysis of the replication of the UV-damaged plasmid revealed that closed circular replication products were sensitive to T4 endonuclease V (a pyrimidine cyclobutane dimer-specific endonuclease) and that this sensitivity was abolished by treatment of the replicated DNA with E. coli photolyase after replication but before T4 endonuclease treatment. These results demonstrate that these closed circular replication products contain pyrimidine cyclobutane dimers. Density labeling experiments revealed that the majority of plasmid DNA synthesized in vitro in the presence of bromodeoxyuridine triphosphate was hybrid density whether or not the plasmid was treated with UV radiation before replication; therefore, replication of UV-damaged templates appears to occur by the normal semiconservative mechanism. All of these data suggest that replication of UV-damaged templates occurs in vitro as it does in vivo and that this replication results in mutation fixation.     

Linker insertion mutants of simian virus 40 large T antigen that show trans-dominant interference with wild-type large T antigen map to multiple sites within the T-antigen gene.

Linker insertion mutants affecting the simian virus 40 (SV40) large tumor (T) antigen were constructed by inserting a 12-base-pair oligonucleotide linker into restriction endonuclease cleavage sites located within the early region of SV40. One mutant, with the insertion at amino acid 5, was viable in CV-1p and BSC-1 cells, indicating that sequences very close to the amino terminus of large T could be altered without affecting the lytic infection cycle of SV40. All other mutants affecting large T were not viable. In complementation assays between the linker insertion mutants and either a late-gene mutant, dlBC865, or a host range/helper function (hr/hf) mutant, dlA2475, delayed complementation was seen with the 6 of the 10 nonviable mutants. Of these 10 mutants, 5 formed plaques 3 to 4 days later than in control complementations, while complementation by one of the mutants, inA2827, with an insertion at amino acid 520, was delayed more than 1 week. Most mutants which showed delayed complementation replicated less well in Cos-1 cells than did a control mutant, dlA1209, which produced no T antigen. The replication of inA2827(aa520) was reduced by more than 90%. Similar interference with viral DNA replication was seen when CV-1, HeLa, or 293 cells were cotransfected with an origin-defective plasmid encoding wild-type large T antigen and with inA2827(aa520). Only one of the mutant T antigens, inA2807(aa303), was unstable. These results indicate that some of the mutant T antigens interfered with functions of wild-type T required for viral DNA replication. However, not all of the mutants which showed delayed complementation also showed interference with viral DNA replication. This indicates that mutant large T antigens may interfere trans dominantly with multiple activities of wild-type large T antigen.     

Identification of rad27 Mutations That Confer Differential Defects in Mutation Avoidance, Repeat Tract Instability, and Flap Cleavage

In eukaryotes, the nuclease activity of Rad27p (Fen1p) is thought to play a critical role in lagging-strand DNA replication by removing ribonucleotides present at the 5' ends of Okazaki fragments. Genetic analysis of Saccharomyces cerevisiae also has identified a role for Rad27p in mutation avoidance. rad27Delta mutants display both a repeat tract instability phenotype and a high rate of forward mutations to canavanine resistance that result primarily from duplications of DNA sequences that are flanked by direct repeats. These observations suggested that Rad27p activities in DNA replication and repair could be altered by mutagenesis and specifically assayed. To test this idea, we analyzed two rad27 alleles, rad27-G67S and rad27-G240D, that were identified in a screen for mutants that displayed repeat tract instability and mutator phenotypes. In chromosome stability assays, rad27-G67S strains displayed a higher frequency of repeat tract instabilities relative to CAN1 duplication events; in contrast, the rad27-G240D strains displayed the opposite phenotype. In biochemical assays, rad27-G67Sp displayed a weak exonuclease activity but significant single- and double-flap endonuclease activities. In contrast, rad27-G240Dp displayed a significant double-flap endonuclease activity but was devoid of exonuclease activity and showed only a weak single-flap endonuclease activity. Based on these observations, we hypothesize that the rad27-G67S mutant phenotypes resulted largely from specific defects in nuclease function that are important for degrading bubble intermediates, which can lead to DNA slippage events. The rad27-G240D mutant phenotypes were more difficult to reconcile to a specific biochemical defect, suggesting a structural role for Rad27p in DNA replication and repair. Since the mutants provide the means to relate nuclease functions in vitro to genetic characteristics in vivo, they are valuable tools for further analyses of the diverse biological roles of Rad27p.     

Mutagenic action of alkylating agents on prophage lambda

The lethal and mutagenic effects of 7 alkylating agents: N-nitroso-N-methylurea (NMU), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), nitrogen mustard (HN2), mitomycin C (MC), bifunctional acridine mustard (AM)--and of cyanate (KNCO) on heat inducible lambda cI857 prophage were studied. After treatment of lysogenic cells with mutagens, prophage was heat-induced either immediately or after 90 min incubation in nutrient broth and c mutants forming clear plaques at 32 degrees C were scored. NMU (0.02 M) when immediately induced with heat, induces c mutants very efficiently (maximal yield 10%) not only in the wild-type cells but also in repair-deficient mutants recA13, lexA102, uvrA6 umuC36, recF143, xthA9, polA1, uvrD3 and uvrD502. These data show that NMU-induced mutations are fixed as replication errors due to mispairing modified bases. After delayed heat induction, the prophage survival enhances and the frequency of c mutations declines considerably in host cells of all repair genotypes tested. Carbamoylation is not involved in the mutagenic action of NMU, because KNCO (0.02 M) has a very slight lethal effect and does not induce mutations. MNNG (100 micrograms/ml) and EMS (0.1 M) also induce mutations by replicative mechanism, because maximal yield of c mutations does not depend on RecA+ and is about 15 and 2%, respectively. MMS is a mutagen of the repair type, since its mutagenic action is suppressed by recA mutation of the host. NH2 only inactivates prophage, but does not induce mutations. MC (50 micrograms/ml) and AM (150 micrograms/ml) induce mutations rather inefficiently (the maximal yield 0.1 and 0.3%, respectively) both in recA+ and recA- hosts. The mutagenic action of these agents is probably due to intercalation.     

A mutational analysis of the yeast proliferating cell nuclear antigen indicates distinct roles in DNA replication and DNA repair.

The saccharomyces cerevisiae proliferating cell nuclear antigen (PCNA), encoded by the POL30 gene, is essential for DNA replication and DNA repair processes. Twenty-one site-directed mutations were constructed in the POL30 gene, each mutation changing two adjacently located charged amino acids to alanines. Although none of the mutant strains containing these double-alanine mutations as the sole source of PCNA were temperature sensitive or cold sensitive for growth, about a third of the mutants showed sensitivity to UV light. Some of those UV-sensitive mutants had elevated spontaneous mutation rates. In addition, several mutants suppressed a cold-sensitive mutation in the CDC44 gene, which encodes the large subunit of replication factor C. A cold-sensitive mutant, which was isolated by random mutagenesis, showed a terminal phenotype at the restrictive temperature consistent with a defect in DNA replication. Several mutant PCNAs were expressed and purified from Escherichia coli, and their in vitro properties were determined. The cold-sensitive mutant (pol30-52, S115P) was a monomer, rather than a trimer, in solution. This mutant was deficient for DNA synthesis in vitro. Partial restoration of DNA polymerase delta holoenzyme activity was achieved at 37 degrees C but not at 14 degrees C by inclusion of the macromolecular crowding agent polyethylene glycol in the assay. The only other mutant (pol30-6, DD41,42AA) that showed a growth defect was partially defective for interaction with replication factor C and DNA polymerase delta but completely defective for interaction with DNA polymerase epsilon. Two other mutants sensitive to DNA damage showed no defect in vitro. These results indicate that the latter mutants are specifically impaired in one or more DNA repair processes whereas pol30-6 and pol30-52 mutants show their primary defects in the basic DNA replication machinery with probable associated defects in DNA repair. Therefore, DNA repair requires interactions between repair-specific protein(s) and PCNA, which are distinct from those required for DNA replication.     

Physical and Genetic Characterization of Deletion Mutants of Simian Virus 40 Constructed In Vitro

Mutants of simian virus 40 (SV40), with deletions ranging in size from fewer than 3 to 750 base pairs located throughout the SV40 genome, were obtained by infecting CV-1P cells with linear SV40 DNA and DNA of an appropriate helper virus. The linear DNA was obtained by complete cleavage of closed circular DNA with Hae II or Bam HI endonuclease or partial cleavage with either Hae III endonuclease or nuclease S1, followed, in some cases, by mild digestion with phage lambda 5' -exonuclease. The following mutants with deletions in the late region of the SV40 genome were obtained and characterized. Ten, containing deletions at the Hae II endonuclease site (map location 0.83), define a new genetic complementation group, E, grow extremely slowly without helper virus, and cause alterations only in VP2. Two mutants with deletions in the region 0.92 to 0.945 affect both VP2 and VP3, demonstrating that VP3 shares sequences with the C-terminal portion of VP2. The mutant with a deletion at 0.93 is the first deletion mutant in the D complementation group and is also temperature sensitive; the mutant with a deletion at 0.94 is viable and grows normally. Three mutants with deletions at the EcoRI endonuclease site (0/1.0) and eleven with deletions at the BamHI endonuclease site (0.15) fall into the B/C complementation group. Six additional mutants with deletions at the BamHI endonuclease site are viable, growing more slowly than wild type. VP1 is the only polypeptide affected by mutants in the B/C group. A mutant with a deletion of the region 0.72 to 0.80 has a polar effect, failing to express the E, D, and B/C genes. Mutants with deletions in the early region (0.67 counterclockwise to 0.17) at 0.66 to 0.59, 0.48, 0.47, 0.33, and 0.285 to 0.205 are all members of the A complementation group. Thus, the A gene is the only viral gene in the early region whose expression is necessary for productive infection of permissive cells. Since mutants with deletions in the region 0.59 to 0.54 are viable, two separate regions are essential for expression of the gene A function: 0.66 to 0.59 and 0.54 to 0.21. Mutants with deletions at 0.21 and 0.18 are viable. Approximate map locations of SV40 genes and possible models for their regulation are discussed.     

Isolation of deletion and substitution mutants of adenovirus type 5

The infectivity of adenovirus type 5 DNA can be increased to about 5 x 10³ plaque-forming units per μg DNA if the DNA is isolated as a DNA-protein complex. Utilizing this improved infectivity, a method was developed for the selection of mutants lacking restriction endonuclease cleavage sites. The procedure involves three steps. First, the DNA-protein complex is cleaved with a restriction endonuclease. The Eco RI restriction endonuclease was used here. It cleaves adenovirus type 5 DNA to produce three fragments: fragment A (1–76 map units), fragment C (76–83 map units) and fragment B (10–83 map units). Second, the mixture of fragments is rejoined by incubating with DNA ligase, and, third, the modified DNA is used to infect cells in a DNA plaque assay. Mutants were obtained which lacked the endonuclease cleavage site at 0.83 map units. Such mutant DNAs were selected by this procedure because they were cleaved by the Eco RI endonuclease to produce only two fragments: a normal A fragment and a fused B/C fragment. These two fragments could be rejoined to produce a viable DNA molecule as a result of a bimolecular reaction with one ligation event; this exerted a strong selection for such molecules since a trimolecular reaction (keeping the C fragment in its proper orientation) and two ligation events were required to regenerate a wild-type molecule. The alterations resulting in the loss of the Eco RI endonuclease cleavage site at 0.83 map units include both deletion and substitution mutations. The inserted sequences in the substitution mutations are cellular in origin.