Biased hypermutation and other genetic changes in defective measles viruses in human brain infections

We assessed the alterations of viral gene expression occurring during persistent infections by cloning full-length transcripts of measles virus (MV) genes from brain autopsies of two subacute sclerosing panencephalitis patients and one measles inclusion body encephalitis (MIBE) patient. the sequence of these MV genes revealed that, most likely, almost 2% of the nucleotides were mutated during persistence, and 35% of these differences resulted in amino acid changes. One of these nucleotide substitutions and one deletion resulted in alteration of the reading frames of two fusion genes, as confirmed by in vitro translation of synthetic mRNAs. One cluster of mutations was exceptional; in the matrix gene of the MIBE case, 50% of the U residues were changed to C, which might result from a highly biased copying event exclusively affecting this gene. We propose that the cluster of mutations in the MIBE case, and other combinations of mutations in other cases, favored propagation of MV infections in brain cells by conferring a selective advantage to the mutated genomes.

Measles virus editing provides an additional cysteine-rich protein

The measles virus (MV) phosphoprotein (P) gene encodes two known proteins, P (Mr approximately 70,000), involved in viral transcription, and, in a different reading frame, C (Mr approximately 20,000). By a combination of cDNA cloning, cDNA and RNA sequencing, and in vitro translation, we demonstrate here that the MV P gene also expresses a third product (Mr approximately 46,000) containing the amino-terminal region of P but a different, cysteine-rich carboxy-terminal motif. This third protein is translated from mRNAs in which one G residue has been inserted after three genomically encoded Gs, a modification found in about 50% of the P mRNAs. A smaller fraction of transcripts contain several additional G residues.

Sequence divergence of measles virus haemagglutinin during natural evolution and adaptation to cell culture

Phylogenetic analysis of the sequence of the H gene of 75 measles virus (MV) strains (32 published and 43 new sequences) was carried out. The lineage groups described from comparison of the nucleotide sequences encoding the C-terminal regions of the N protein of MV were the same as those derived from the H gene sequences in almost all cases. The databases document a number of distinct genotype switches that have occurred in Madrid (Spain). Well-documented is the complete replacement of lineage group C2, the common European genotype at that time, with that of group D3 around the autumn of 1993. No further isolations of group C2 took place in Madrid after this time. The rate of mutation of the H gene sequences of MV genotype D3 circulating in Madrid from 1993 to 1996 was very low (5 x 10(-4) per annum for a given nucleotide position). This is an order of magnitude lower than the rates of mutation observed in the HN genes of human influenza A viruses. The ratio of expressed over silent mutations indicated that the divergence was not driven by immune selection in this gene. Variations in amino acid 117 of the H protein (F or L) may be related to the ability of some strains to haemagglutinate only in the presence of salt. Adaptation of MV to different primate cell types was associated with very small numbers of mutations in the H gene. The changes could not be predicted when virus previously grown in human B cell lines was adapted to monkey Vero cells. In contrast, rodent brain-adapted viruses displayed a lot of amino acid sequence variation from normal MV strains. There was no convincing evidence for recombination between MV genotypes.

Altered transcription of a defective measles virus genome derived from a diseased human brain

Measles virus (MV) is a negative strand RNA virus which usually causes acute disease, but in rare cases its persistence in the human brain induces the lethal disease subacute sclerosing panencephalitis (SSPE). The transcription of MV and of a defective MV derived from autopsy material of a SSPE case was studied in cultured cells. In the lytic infection the levels of the MV mRNAs decreased progressively with the distance of the six cognate genes from the 3' end of the genome, reflecting transcriptional attenuation at every gene junction. Transcripts covering two or three adjacent genes accounted for up to 20% of single gene transcripts; incidentally the MV intergenic transcription signals were found to be less conserved than the analogous signals of other negative strand RNA viruses. Although the analysed SSPE-derived defective MV showed a localized transcription defect at the phosphoprotein--matrix gene junction (substitution of the mRNAs by readthrough transcripts), the corresponding intergenic 'consensus' sequence and the surrounding nucleotides were not altered. This implies that factor(s) involved in the transcription of this defective SSPE virus fail to recognize this particular signal sequence, a constellation which in this and other cases might be causally related to the development of MV persistence.

Mutated and hypermutated genes of persistent measles viruses which caused lethal human brain diseases

Persistent measles viruses (MVs) causing lethal human brain diseases are defective, and the structure of several mutated matrix genes has been elucidated previously. The present study of four persistent MVs revealed a high number of differences from a consensus sequence also in other genes. Amino acid changes accumulated in the carboxyl terminus of the nucleocapsid protein and in the amino terminus of the phosphoprotein, but did not significantly alter these products, which are implicated in viral replication and transcription. The contrary is true for the envelope glycoproteins: In three of four cases, mutations caused partial deletion of the short intracellular domain of the fusion protein, most likely compromising efficient viral budding. Moreover, in the hemagglutinin gene of a strain showing strongly reduced hemadsorption, 20 clustered A to G mutations, resulting in 16 amino acid changes, were detected. This hypermutation might be due to unwinding modification of a part of the MV RNA genome accidentally present in a double-stranded form. Finally, we classified four lytic and seven persistent MV strains on the basis of their sequences. Surprisingly, the four lytic viruses considered belong to the same class. The persistent viruses form more loosely defined groups, which all differ from the vaccine strain Edmonston.

Receptor usage and differential downregulation of CD46 by measles virus wild-type and vaccine strains

Recently, two cell surface molecules, CD46 and moesin, have been found to be functionally associated with measles virus (MV) infectivity of cells. We investigated the receptor usage of MV wild-type, subacute sclerosing panencephalitis, and vaccine strains and their effect on the down-regulation of CD46 after infection. We found that the infection of human cell lines with all 19 MV strains tested was inhibitable with antibodies against CD46. In contrast, not all strains of MV led to the downregulation of CD46 following infection. The group of CD46 non-downregulating strains comprised four lymphotropic wild-type isolates designated AB, DF, DL, and WTF. Since the downregulation of CD46 is caused by interaction with newly synthesized MV hemagglutinin (MV-H), we tested the capability of recombinant MV-H proteins to downregulate CD46. Recombinant MV-H proteins of MV strains Edmonston, Halle, and CM led to the down-regulation of CD46, whereas those of DL and WTF did not. This observed differential downregulation by different MV strains has profound consequences, since lack of CD46 on the cell surface leads to susceptibility of cells to complement lysis. These results suggest that lymphotropic wild-type strains of MV which do not downregulate CD46 may have an advantage for replication in vivo. The relatively weak immune response against attenuated vaccine strains of MV compared with wild-type strains might be related to this phenomenon.

Temporal and geographical distribution of measles virus genotypes

The nucleotide sequence encoding the C terminus of the nucleocapsid protein of measles virus (MV) is the most variable in the genome. The sequence of this region is reported for 21 new MV strains and for virus RNA obtained from cases of subacute panencephalitis (SSPE) tissue. The nucleotide sequence of a total of 65 MV strains has been analysed using the CLUSTAL program to determine the relationships between the strains. An unrooted tree shows that eight different genotypes can be discerned amongst the sequences analysed so far. The data show that the C-terminal coding sequence of the nucleocapsid gene, although highly variable between strains, is stable in a given strain and does not appear to diverge in tissue culture. It therefore provides a good 'signature' sequence for specific genotypes. The sequence of this region can be used to discriminate new imported viruses from old 'endemic' strains of MV in a geographical area. The different genotypes are not geographically restricted although some appear to be the mainly 'endemic' types in large areas of the world. In global terms there appears to be at least four cocirculating genotypes of MV. The low level of divergence in the Edmonston lineage group isolated before 1970 indicates that some isolates are probably laboratory contaminants. This applies to some SSPE isolates such as the Hallé, Mantooth and Horta-Barbosa strains as well as some wild-type isolates from that period.

Expression of defective measles virus genes in brain tissues of patients with subacute sclerosing panencephalitis

The persistence of measles virus in selected areas of the brains of four patients with subacute sclerosing panencephalitis (SSPE) was characterized by immunohistological and biochemical techniques. The five measles virus structural proteins were never simultaneously detectable in any of the brain sections. Nucleocapsid proteins and phosphoproteins were found in every diseased brain area, whereas hemagglutinin protein was detected in two cases, fusion protein was detected in three cases, and matrix protein was detected in only one case. Also, it could be shown that the amounts of measles virus RNA in the brains differed from patient to patient and in the different regions investigated. In all patients, plus-strand RNAs specific for these five viral genes could be detected. However, the amounts of fusion and hemagglutinin mRNAs were low compared with the amounts in lytically infected cells. The presence of particular measles virus RNAs in SSPE-infected brains did not always correlate with mRNA activity. In in vitro translations, the matrix protein was produced in only one case, and the hemagglutinin protein was produced in none. These results indicate that measles virus persistence in SSPE is correlated with different defects of several genes which probably prevent assembly of viral particles in SSPE-infected brain tissue.

Accumulated measles virus mutations in a case of subacute sclerosing panencephalitis: interrupted matrix protein reading frame and transcription alteration

Subacute sclerosing panencephalitis (SSPE) is a fatal disease affecting the human central nervous system several years after acute measles infection. Measles virus (MV) genomes replicating in SSPE brains do not give rise to budding particles and present various defects in gene expression, mostly concerning the matrix (M) protein. For one SSPE case (K), shown previously to be devoid of M protein expression, we examined here in detail the features involved in this defect. In the brain of patient K the normal, monocistronic MV M mRNA was completely substituted by a bicistronic RNA containing the coding sequence of the preceding phosphoprotein (P) gene in addition to the M coding sequence. Analysis of the P-M intercistronic region by direct cDNA sequencing showed that the consensus sequence at this RNA processing site was unaltered but revealed several distant point mutations. cDNA cloning and sequencing of the entire M coding region established that one of the point mutations leads to a stop codon at triplet 12 of the M reading frame. It is unknown whether this defect, explaining by itself the lack of M protein, is related also to the block of M mRNA formation. In addition we note that as much as 1% of the nucleotides differed between two overlapping clones from the same brain. This high sequence variability could possibly account for the diversity of defects observed in MV gene expression in SSPE brains and may be a general phenomenon associated with RNA virus persistence.

Subacute sclerosing panencephalitis is typically characterized by alterations in the fusion protein cytoplasmic domain of the persisting measles virus

Our recent extensive analysis of three cases of subacute sclerosing panencephalitis (SSPE) revealed intriguing genetic defects in the persisting measles virus (MV): the fusion (F) genes encoded truncated cytoplasmic F protein domains (Cattaneo et al., Virology 173, 415-425, 1989). Now this MV genomic region has been investigated in eight additional SSPE cases by PCR amplification, replacement cloning into a vector containing the F gene of a lytic MV, in vitro expression, and sequencing. In all cases at least part of the clones showed mutations leading to F protein truncations, elongation, or nonconservative amino acid replacements. It is proposed that alteration of the F protein cytoplasmic domain may play a critical role in the development of SSPE.

Identification of several different lineages of measles virus

The sequences of a region of the nucleocapsid protein gene, between nucleotides 1231 and 1686, encoding the C-terminal 151 amino acid residues of the nucleocapsid protein have been determined for 16 strains of measles virus. Analysis of this region showed that it is highly divergent (up to 7.2% divergence in the nucleotide sequence and 10.6% divergence in the amino acid sequence between most distant strains) and that several lineages of measles virus can be found to co-circulate at a given time. Some of the lineages show geographical restriction. The results for measles virus are similar to those reported for other human paramyxoviruses such as mumps virus, parainfluenza type 3 virus and the avian Newcastle disease virus.

Altered ratios of measles virus transcripts in diseased human brains

In rare cases measles virus (MV) induces subacute sclerosing panencephalitis (SSPE) or measles inclusion body encephalitis (MIBE), two lethal diseases of the human central nervous system. MV transcripts present in the brains of two SSPE patients and one MIBE patient were analyzed by quantitative Northern blots. In all three cases the transcripts from the first MV gene were relatively abundant, amounting to about one-tenth of that in lytically infected cells. However, the quantity of transcripts decreased sharply for each subsequent MV gene, arriving at 200-fold lower levels for the fifth MV gene. In comparison gradients of transcript levels are more shallow in either lytically or persistently infected cultured cells, where the transcripts of the fifth MV gene are only about five times less abundant than those of the first. These altered ratios of mRNAs appear to be typical for persistent MV brain infections and most likely lead to reduced expression of the viral envelope proteins, encoded by distal MV genes, at the surface of brain cells. This could account for the lack of viral budding and allow persistent MV infections to elude immune surveillance.

Clonal expansion of hypermutated measles virus in a SSPE brain

Nucleotide sequence analysis was carried out to study genes encoding the matrix (M) protein of measles virus (MV) from several regions of the brain of a case of subacute sclerosing panencephalitis. This analysis revealed the presence of MV with "wild-type" sequences as well as variants which had undergone at least five biased hypermutation events (U to C and A to G in the positive strand sequences). Despite the presence of MV variants with genes encoding the intact matrix protein open reading frame, M protein could not be detected in any of the brain regions. The distribution of virus variants was studied by cDNA cloning and sequence analysis and by in situ hybridization. The hypermutated viruses appeared to expand clonally throughout the brain of patient B.

Generation and properties of measles virus mutations typically associated with subacute sclerosing panencephalitis

Subacute sclerosing panencephalitis (SSPE), a very rare but lethal disease caused by measles viruses (MV) persisting in the human central nervous system (CNS) is characterized by lack of viral budding, reduced expression of the viral envelope proteins and spread of MV genomes through the CNS despite massive immune responses. The five major MV genes from several SSPE cases were cloned and sequenced, the two transmembrane envelope glycoproteins hemagglutinin (H) and fusion protein (F) were expressed and their maturation, cellular localization and functionality analyzed. We conclude that 1) mutations in the MV genes arise not only individually, by errors of the MV polymerase, but also in clusters as hypermutations, presumably due to RNA unwinding/modifying activity altering accidentally formed double-stranded RNA regions, 2) MVs spread in SSPE brains after clonal selection, 3) the MV matrix (M) gene is most heavily mutated and dispensable, 4) the two genes encoding envelope transmembrane proteins give rise to functional but altered proteins (typically F is heavily altered in its cytoplasmic domain), 5) H protein is transported poorly to the cell surface, 6) F and H proteins maintain tightly interdepending fusion functions, presumably to allow local cell fusion and MV ribonucleoprotein (RNP) spread through the CNS.

Transcription inhibition and other properties of matrix proteins expressed by M genes cloned from measles viruses and diseased human brain tissue

Ribonucleoprotein (RNP) cores extracted from virions of wild-type (Edmonston strain) measles virus (MV) or obtained from MV-infected cells (cRNP) were shown to be capable of transcribing RNA in vitro but at relatively low efficiency. The tightly bound matrix (M) protein could be effectively removed from virion RNP (vRNP) and from cRNP by exposure to buffers of high ionic strength (0.5 to 1.0 M KCl) but only at pH 8.0 or higher. The vRNP and cRNP cores complexed with M protein exhibited markedly reduced transcriptional activity at increasing concentrations, whereas vRNP and cRNP cores free of M protein exhibited linear and substantially higher transcriptional activity; these data suggest that M protein is the endogenous inhibitor of MV RNP transcription. M-gene cDNA clones derived from three strains of wild-type (wt) MV and 10 clones from mRNAs isolated from the brain tissue of patients who had died from subacute sclerosing panencephalitis (SSPE) and from measles inclusion body encephalitis (MIBE) were recloned in the pTM-1 expression vector driven by the bacteriophage T7 RNA polymerase expressed by a coinfecting vaccinia virus recombinant. All 10 mutant SSPE and MIBE clones expressed in vitro and in vivo M proteins that reacted with monospecific anti-M polyclonal antibody and migrated on polyacrylamide gels to positions identical to or only slightly different from those of the M proteins expressed by wt MV clones. When reconstituted with cRNP cores, the three expressed wt M proteins and 6 of the 10 mutant-expressed M proteins showed equivalent capacity to down-regulate MV transcription. Three of the M proteins from SSPE clones and one from the MIBE clone showed little or no capacity to down-regulate transcription when reconstituted with cRNP cores. The only plausible explanations for loss of transcription inhibition activity by the four SSPE/MIBE M proteins were exceedingly high degrees of hypermutations leading to U-->C transitions and cloning-corrected mutations in the initiator codon (ATG-->ACG) of the four M genes. However, only the hypermutated M protein expressed by the MIBE cDNA clone exhibited virtually no capacity to bind cRNP cores in a reconstitution assay. These experiments provide some preliminary data to support the hypothesis that MV encephalitis may result from certain selective mutations in the M gene.

Restricted expression of measles virus proteins in brains from cases of subacute sclerosing panencephalitis

The presence of five structural proteins of measles virus in brain material obtained at autopsy from four patients with subacute sclerosing panencephalitis (SSPE) was examined by immunofluorescence employing monoclonal antibodies. In addition, the humoral immune response against measles virus antigens in serum and cerebrospinal fluid was analysed by immunoprecipitation in combination with gel electrophoresis, revealing a reduced response mainly to the matrix (M) protein. In none of the brain material were all five structural proteins simultaneously detected. Nucleocapsid protein and phosphoprotein were found in every diseased brain area, whereas haemagglutinin (H) protein was detected in two, fusion (F) protein in three and M protein only in one SSPE case. In two cases, variations in the occurrence of H and F proteins could be observed between regions displaying different degrees of neuropathological changes. No correlation was observed between the humoral immune response and the immunohistological findings. These data support the hypothesis of a restricted synthesis of measles virus proteins, in particular the envelope and M proteins, in SSPE.

Antibody-induced restriction of viral gene expression in measles encephalitis in rats

After infection with the neurotropic CAM/RBH measles virus (MV) strain, newborn Lewis rats succumb to an acute necrotizing encephalopathy. Passive transfer of neutralizing monoclonal antibodies directed against MV hemagglutinin prevented this disease process. Instead, either an antibody-induced acute or subacute measles encephalitis developed after a prolonged incubation period with a restricted expression of MV structural proteins. The molecular biological analysis of MV gene expression in brain tissue of rats treated with MV-neutralizing antibodies revealed a transcriptional restriction of viral mRNAs, particularly for the envelope proteins, leading to a steep expression gradient. Based on in situ hybridization, it was concluded that the efficiency of transcription of viral genes at the single-cell level is reduced compared with that of controls. Passive immunization with monoclonal antibodies directed against other MV structural proteins proved to be ineffective. Similar results were obtained in MV-infected weanling Brown Norway rats. These rats developed a clinically silent encephalitis in the presence of high titers of neutralizing antibodies. In such animals, a pronounced attenuation of the viral gene transcription was observed. These findings indicated that neutralizing antibodies directed against a restricted set of specific antigenic sites on the viral hemagglutinin protein expressed on cell membranes exert a modulating effect on the viral gene expression at the level of transcription. This phenomenon contributes to the switch from the acute cytopathic effect to a persistent infection in the central nervous system.

Antigenic determinants of measles virus hemagglutinin associated with neurovirulence

The biological activity of monoclonal antibodies specific for the hemagglutinin protein of measles virus strain CAM recognizing six epitope groups according to their binding properties to measles virus strain CAM/R401 was investigated in vivo in our rat model of measles encephalitis. When injected intraperitoneally into measles virus-infected suckling rats, some monoclonal antibodies modified the disease process and prevented the necrotizing encephalopathy seen in untreated animals. The analysis of measles virus brain isolates revealed emergence of variants that resisted neutralization with the passively transferred selecting monoclonal antibody but not with other monoclonal antibodies. Monoclonal antibody escape mutants were also isolated in vitro, and their neurovirulence varied in the animal model. Sequence data from the hemagglutinin gene of measles virus localize a major antigenic surface determinant of the hemagglutinin protein between amino acid residues 368 and 396, which may be functionally important for neurovirulence. The data indicate that the interaction of antibodies with the measles virus H protein plays an important role in the selection of neurovirulent variants. These variants have biological properties different from those of the parent CAM virus.

Restriction of measles virus gene expression in acute and subacute encephalitis of Lewis rats

Measles virus (MV) replication in brain tissue of Lewis rats with acute (AE) and subacute (SAME) encephalitis was characterized by biochemical techniques. Messenger RNAs specific for measles virus nucleocapsid (N), phospho (P)-, matrix (M), fusion (F), and haemagglutinin (H) protein were detected in all brain extracts examined. The quantity of the individual MV mRNA species was quite different in comparison to lytically infected Vero cells. A steep gradient of MV transcripts was found in brain tissue which is most likely due to strongly attenuated transcription of mRNAs along the viral genome, representing particularly low transcription of the glycoprotein genes. In addition, in vitro translation assays only revealed synthesis of N and P protein in consistent fashion. The mRNAs for the glycoproteins did not direct the synthesis of detectable viral proteins whereas the M mRNA revealed some activity in animals with AE. The data indicate a strong restriction of the MV envelope gene expression in infected brain tissue, which is independent of the incubation time and type of the central nervous system (CNS) disease. This phenomenon which is similar to the findings observed in measles inclusion body encephalitis and subacute sclerosing panencephalitis suggest that host factors may initially be responsible for the initiation of transcriptional and translational alterations.

Cloning of DNA corresponding to four different measles virus genomic regions

Hybrid plasmids containing sequences corresponding to four different regions of the measles virus genome inserted in pBR322 were obtained by use of polyadenylated 50 S viral RNA as template for reverse transcription. One class of plasmids contains inserts corresponding to the 3' terminal region of the virus genome. The sequence of one of these inserts (605 nucleotides) partially overlaps with the cloned cDNA sequence corresponding to a part of the nucleocapsid protein (N) mRNA (M. Gorecki and S. Rozenblatt (1980). Proc. Natl. Acad. Sci. USA 77, 3686-3690). This insert region shows only one long open reading frame defining the N-terminal part of the nucleocapsid protein. The nucleocapsid protein mRNA starts at about 60 nucleotides from the genome end as revealed by nuclease S1 mapping. Three other classes of plasmid clones contain inserts derived from unidentified regions of the viral genome; they hybridize with viral mRNA species less abundant than those from which cDNA clones have been isolated so far (S. Rozenblatt, C. Gesang, V. Lavie, and F. S. Neumann (1982) J. Virol. 42, 790-797.

Measles virus gene expression in subacute sclerosing panencephalitis

RNA was extracted from the diseased brain of a case of human subacute sclerosing panencephalitis (SSPE) and analysed for the expression of measles-specific RNA. Measles virus-specific mRNAs were present, but the amount of matrix (M) protein mRNA was greatly reduced in comparison to lytically infected cells and phospho- (P) protein mRNA was hardly detectable whereas the level of the corresponding intermediate-sized (is-) RNA was greatly increased. RNA obtained from the human brain was also translated in vitro and measles virus nucleocapsid and P protein was produced. However, in marked contrast to control reactions M protein was not detected in the products formed by translation in vitro. These results indicate an impaired measles virus M protein mRNA synthesis in infected brain tissue.

A high fatigue resistant, photoswitchable fluorescent spiropyran–polyoxometalate–BODIPY single-molecule

In the reported spiropyran/polyoxometalate/BODIPY triad, the fluorescence of the BODIPY can be highly reversibly quenched by the activation of the photochromic spiropyran.

Restriction of measles virus gene expression in measles inclusion body encephalitis

Measles virus (MV) infection in brain tissue of a patient with measles inclusion body encephalitis was characterized by immunologic and biochemical techniques. Of the five major structural proteins of MV, only nucleocapsid (N) protein and phosphoprotein (P protein) were consistently detected in diseased brain areas. In contrast, hemagglutinin protein was seen only occasionally, and no membrane and fusion proteins were found in any of the sections studied. Messenger RNAs (mRNAs) specific for these five viral proteins were detected in all brain extracts examined; however, the mRNAs for the envelope proteins were clearly underrepresented in comparison with lytically infected cells. Only the mRNAs for N and P proteins appeared active in in vitro translations. These findings indicate quantitative differences in the pattern of mRNA expression in brain tissue and a restricted expression of MV envelope proteins in infected cells as observed in subacute sclerosing panencephalitis.

Intracellular forms of adenovirus DNA: integrated form of adenovirus DNA appears early in productive infection

In KB cells productively infected with adenovirus type 2, alkali-stable greater than 100S and 40-100S viral DNAs are synthesized starting 2-4 hr postinfection, i.e., before unit length (34 S) viral DNA is made. The amount of greater than 100S and 40-100S viral DNA increases when 34S viral DNA synthesis begins, and at 16-18 hr postinfection, the 40-100S viral DNA represents 5-20% of the total intracellular viral DNA. The 40-100S viral DNA is synthesized throughout infection. Part of the 40-100S DNA synthesized 5-8 hr postinfection has a density in alkaline CsCl gradients intermediate between those of viral and cellular DNAs. This finding indicates that newly synthesized viral DNA is covalently linked to cellular DNA. Viral sequences can be excised from the cellular DNA of infected cells with the EcoRI restriction endonuclease. Fragments of viral DNA are detected in polyacrylamide-agarose gels by DNA-DNA hybridization, and these fragments correspond in size to most of the known EcoRI fragments of adenovirus 2 DNA. Viral DNA sequences in size-classes between the EcoRI-A and -C fragments are also found and probably represent viral DNA linked to cellular sequences.