Cryo-EM reveals a novel octameric integrase structure for betaretroviral intasome function.

Retroviral integrase catalyses the integration of viral DNA into host target DNA, which is an essential step in the life cycle of all retroviruses. Previous structural characterization of integrase-viral DNA complexes, or intasomes, from the spumavirus prototype foamy virus revealed a functional integrase tetramer, and it is generally believed that intasomes derived from other retroviral genera use tetrameric integrase. However, the intasomes of orthoretroviruses, which include all known pathogenic species, have not been characterized structurally. Here, using single-particle cryo-electron microscopy and X-ray crystallography, we determine an unexpected octameric integrase architecture for the intasome of the betaretrovirus mouse mammary tumour virus. The structure is composed of two core integrase dimers, which interact with the viral DNA ends and structurally mimic the integrase tetramer of prototype foamy virus, and two flanking integrase dimers that engage the core structure via their integrase carboxy-terminal domains. Contrary to the belief that tetrameric integrase components are sufficient to catalyse integration, the flanking integrase dimers were necessary for mouse mammary tumour virus integrase activity. The integrase octamer solves a conundrum for betaretroviruses as well as alpharetroviruses by providing critical carboxy-terminal domains to the intasome core that cannot be provided in cis because of evolutionarily restrictive catalytic core domain-carboxy-terminal domain linker regions. The octameric architecture of the intasome of mouse mammary tumour virus provides new insight into the structural basis of retroviral DNA integration.

Impact of combination antiretroviral therapy in the NOD.c3c4 mouse model of autoimmune biliary disease.

BACKGROUND & AIMS: The NOD.c3c4 mouse model develops autoimmune biliary disease characterized by spontaneous granulomatous cholangitis, antimitochondrial antibodies and liver failure. This model for primary biliary cirrhosis (PBC) has evidence of biliary infection with mouse mammary tumour virus (MMTV), suggesting that the virus may have a role in cholangitis development and progression of liver disease in this mouse model. We tested the hypothesis that MMTV infection is associated with cholangitis in the NOD.c3c4 mouse model by investigating whether antiretroviral therapy impacts on viral levels and liver disease. METHODS: NOD.c3c4 mice were treated with combination antiretroviral therapy. Response to treatment was studied by measuring MMTV RNA in the liver, liver enzyme levels in serum and liver histology using a modified Ishak score. RESULTS: Combination therapy with the reverse transcriptase inhibitors, tenofovir and emtricitabine, resulted in a significant reduction in serum liver enzyme levels, attenuation of cholangitis and decreased MMTV levels in the livers of NOD.c3c4 mice. Furthermore, treatment with the retroviral protease inhibitors, lopinavir and ritonavir, in addition to the reverse transcriptase inhibitors, resulted in further decrease in MMTV levels and attenuation of liver disease in this model. CONCLUSIONS: The attenuation of cholangitis with regimens containing the reverse transcriptase inhibitors, tenofovir and emtricitabine, and the protease inhibitors, lopinavir and ritonavir, suggests that retroviral infection may play a role in the development of cholangitis in this model.

Assembly mechanisms of RNA pseudoknots are determined by the stabilities of constituent secondary structures.

Understanding how RNA molecules navigate their rugged folding landscapes holds the key to describing their roles in a variety of cellular functions. To dissect RNA folding at the molecular level, we performed simulations of three pseudoknots (MMTV and SRV-1 from viral genomes and the hTR pseudoknot from human telomerase) using coarse-grained models. The melting temperatures from the specific heat profiles are in good agreement with the available experimental data for MMTV and hTR. The equilibrium free energy profiles, which predict the structural transitions that occur at each melting temperature, are used to propose that the relative stabilities of the isolated helices control their folding mechanisms. Kinetic simulations, which corroborate the inferences drawn from the free energy profiles, show that MMTV folds by a hierarchical mechanism with parallel paths, i.e., formation of one of the helices nucleates the assembly of the rest of the structure. The SRV-1 pseudoknot, which folds in a highly cooperative manner, assembles in a single step in which the preformed helices coalesce nearly simultaneously to form the tertiary structure. Folding occurs by multiple pathways in the hTR pseudoknot, the isolated structural elements of which have similar stabilities. In one of the paths, tertiary interactions are established before the formation of the secondary structures. Our work shows that there are significant sequence-dependent variations in the folding landscapes of RNA molecules with similar fold. We also establish that assembly mechanisms can be predicted using the stabilities of the isolated secondary structures.

Inhibition of peroxisome proliferator-activated receptor gamma increases estrogen receptor-dependent tumor specification.

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor that regulates gene transcription associated with intermediary metabolism, adipocyte differentiation, and tumor suppression and proliferation. To understand the role of PPARgamma in tumorigenesis, transgenic mice were generated with mammary gland-directed expression of the dominant-negative transgene Pax8PPARgamma. Transgenic mice were phenotypically indistinguishable from wild-type (WT) mice, but mammary epithelial cells expressed a greater percentage of CD29(hi)/CD24(neg), CK5(+), and double-positive CK14/CK18 cells. These changes correlated with reduced PTEN and increased Ras and extracellular signal-regulated kinase (ERK) and AKT activation. Although spontaneous tumorigenesis did not occur, transgenic animals were highly susceptible to progestin/7,12-dimethylbenz(a)anthracene-induced mammary carcinogenesis, which in contrast to WT mice resulted in a high tumor multiplicity and, most importantly, in the appearance of predominantly estrogen receptor alpha-positive (ER(+)) ductal adenocarcinomas. Tumors expressed a similar PTEN(lo)/pERK(hi)/pAKT(hi) phenotype as mammary epithelium and exhibited high activation of estrogen response element-dependent reporter gene activity. Tumorigenesis in MMTV-Pax8PPARgamma mice was insensitive to the chemopreventive effect of a PPARgamma agonist but was profoundly inhibited by the ER antagonist fulvestrant. These results reveal important new insights into the previously unrecognized role of PPARgamma in the specification of mammary lineage and the development of ER(+) tumors.

HIV protease cleaves poly(A)-binding protein.

The PABP [poly(A)-binding protein] is able to interact with the 3' poly(A) tail of eukaryotic mRNA, promoting its translation. Cleavage of PABP by viral proteases encoded by several picornaviruses and caliciviruses plays a role in the abrogation of cellular protein synthesis. We report that infection of MT-2 cells with HIV-1 leads to efficient proteolysis of PABP. Analysis of PABP integrity was carried out in BHK-21 (baby-hamster kidney) and COS-7 cells upon individual expression of the protease from several members of the Retroviridae family, e.g. MoMLV (Moloney murine leukaemia virus), MMTV (mouse mammary tumour virus), HTLV-I (human T-cell leukaemia virus type I), SIV (simian immunodeficiency virus), HIV-1 and HIV-2. Moreover, protease activity against PABP was tested in a HeLa-cell-free system. Only MMTV, HIV-1 and HIV-2 proteases were able to cleave PABP in the absence of other viral proteins. Purified HIV-1 and HIV-2 proteases cleave PABP1 directly at positions 237 and 477, separating the two first RNA-recognition motifs from the C-terminal domain of PABP. An additional cleavage site located at position 410 was detected for HIV-2 protease. These findings indicate that some retroviruses may share with picornaviruses and caliciviruses the capacity to proteolyse PABP.

Amino acid preferences for a critical substrate binding subsite of retroviral proteases in type 1 cleavage sites.

The specificities of the proteases of 11 retroviruses representing each of the seven genera of the family Retroviridae were studied using a series of oligopeptides with amino acid substitutions in the P2 position of a naturally occurring type 1 cleavage site (Val-Ser-Gln-Asn-Tyr Pro-Ile-Val-Gln; the arrow indicates the site of cleavage) in human immunodeficiency virus type 1 (HIV-1). This position was previously found to be one of the most critical in determining the substrate specificity differences of retroviral proteases. Specificities at this position were compared for HIV-1, HIV-2, equine infectious anemia virus, avian myeloblastosis virus, Mason-Pfizer monkey virus, mouse mammary tumor virus, Moloney murine leukemia virus, human T-cell leukemia virus type 1, bovine leukemia virus, human foamy virus, and walleye dermal sarcoma virus proteases. Three types of P2 preferences were observed: a subgroup of proteases preferred small hydrophobic side chains (Ala and Cys), and another subgroup preferred large hydrophobic residues (Ile and Leu), while the protease of HIV-1 preferred an Asn residue. The specificity distinctions among the proteases correlated well with the phylogenetic tree of retroviruses prepared solely based on the protease sequences. Molecular models for all of the proteases studied were built, and they were used to interpret the results. While size complementarities appear to be the main specificity-determining features of the S2 subsite of retroviral proteases, electrostatic contributions may play a role only in the case of HIV proteases. In most cases the P2 residues of naturally occurring type 1 cleavage site sequences of the studied proteases agreed well with the observed P2 preferences.

The mature reverse transcriptase molecules in virions of mouse mammary tumor virus possess protease-derived sequences.

Our efforts to express in bacteria the enzymatically active reverse transcriptase (RT) of mouse mammary tumor virus (MMTV) have shown that the RT is active only after adding 27 amino acid residues, which are derived from the end of the pro gene, to the amino-terminus of the RT (Biochem, J. (1998) 329, 579-587). In the present study we have tested whether the mature RT found in virions is also fused to protease-derived sequences. To this end, we have analyzed the RT molecules in virions of MMTV by using two antisera directed against peptides, derived from either the carboxyl-terminus of MMTV protease or the middle of MMTV RT. The data suggest that the mature RT, located in virions, contains at its amino-terminus sequences from the carboxyl-terminus of the protease protein. This finding supports previous suggestions that MMTV RT is a transframe protein (derived from both pro and pol reading frames of MMTV) and that amino acid residues located at the carboxyl-terminus of the protease have a dual usage as integral parts of both the protease and the RT enzymes.

Matrilysin (matrix metalloproteinase-7) selects for apoptosis-resistant mammary cells in vivo.

Overexpression of the matrix metalloproteinase matrilysin (matrix metalloproteinase-7) in the mouse mammary gland promotes mammary hyperplasia and accelerates the onset of oncogene-induced mammary tumors. In cell culture models, acute exposure of cells coexpressing Fas and Fas ligand (FasL) to matrilysin induces apoptosis, whereas chronic exposure to matrilysin selects for apoptosis-resistant cells. We now demonstrate that matrilysin promotes resistance to apoptosis in vivo. Matrilysin expression increased apoptosis in the involuting mammary gland of mice that had undergone a single pregnancy and lactation cycle. Premature basement membrane disruption was detected in matrilysin-expressing mice, which could account for the increase in apoptosis. However, multiparous mice, in which the involuting mammary epithelial cells have been repeatedly exposed to matrilysin, show a significant decrease in apoptosis. Mammary tissue from multiparous matrilysin-expressing mice showed decreased FasL expression, suggesting that loss of FasL is at least one mechanism of matrilysin-induced resistance to apoptosis. We propose that matrilysin promotes mammary tumor formation by enhancing the selection of cells that are resistant to apoptosis.

Characterization of deoxyuridine 5'-triphosphate nucleotidohydrolase from Trypanosoma cruzi.

We report the cloning and kinetic characterization of Trypanosoma cruzi deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase) whose coding sequence was isolated by genetic complementation in Escherichia coli. The deduced amino acid sequence was similar to Leishmania major dUTPase although it exhibits an amino acid insertion which is sensitive to protease inactivation. The catalytically active species of the enzyme is a dimer and a detailed kinetic characterization showed that it is highly specific for dUTP and dUDP. The general observation that dUTPases from the Trypanosomatidae differ in sequence, conformation and substrate specificity suggests that a different family of dUTPases exists in certain organisms, which may be exploited as drug targets against infectious diseases.

Mode of inhibition of HIV-1 reverse transcriptase by polyacetylenetriol, a novel inhibitor of RNA- and DNA-directed DNA polymerases.

Polyacetylenetriol (PAT), a natural marine product from the Mediterranean sea sponge Petrosia sp., was found to be a novel general potent inhibitor of DNA polymerases. It inhibits equally well the RNA- and DNA-dependent DNA polymerase activities of retroviral reverse transcriptases (RTs) (i.e. of HIV, murine leukaemia virus and mouse mammary tumour virus) as well as cellular DNA polymerases (i.e. DNA polymerases alpha and beta and Escherichia coli polymerase I). A study of the mode and mechanism of the polymerase inhibition by PAT has been conducted with HIV-1 RT. PAT was shown to be a reversible non-competitive inhibitor. PAT binds RT independently and at a site different from that of the primer-template and dNTP substrates with high affinity (K(i)=0.51 microM and K(i)=0.53 microM with dTTP and with dGTP as the variable substrates respectively). Blocking the polar hydroxy groups of PAT has only a marginal effect on the inhibitory capacity, thus hydrophobic interactions are likely to play a major role in inhibiting RT. Preincubation of RT with the primer-template substrate prior to the interaction with PAT reduces substantially the inhibition capacity, probably by preventing these contacts. PAT does not interfere with the first step of polymerization, the binding of RT to DNA, nor does the inhibitor interfere with the binding of dNTP to RT/DNA complex, as evident from the steady-state kinetic study, whereby K(m) remains unchanged. We assume, therefore, that PAT interferes with subsequent catalytic steps of DNA polymerization. The inhibitor may alter the optimal stereochemistry of the polymerase active site relative to the primer terminus, bound dNTP and the metal ions that are crucial for efficient catalysis or, alternatively, may interfere with the thumb sub-domain movement and, thus, with the translocation of the primer-template following nucleotide incorporation.

Polycitone A, a novel and potent general inhibitor of retroviral reverse transcriptases and cellular DNA polymerases.

Polycitone A, an aromatic alkaloid isolated from the ascidian Polycitor sp. exhibits potent inhibitory capacity of both RNA- and DNA-directed DNA polymerases. The drug inhibits retroviral reverse transcriptase (RT) [i.e. of human immunodeficiency virus type 1 (HIV), murine leukaemia virus (MLV) and mouse mammary tumour virus (MMTV)] as efficiently as cellular DNA polymerases (i.e. of both DNA polymerases alpha and beta and Escherichia coli DNA polymerase I). The mode and mechanism of inhibition of the DNA-polymerase activity associated with HIV-1 RT by polycitone A have been studied. The results suggest that the inhibitory capacity of the DNA polymerase activity is independent of the template-primer used. The RNase H function, on the other hand, is hardly affected by this inhibitor. Polycitone A has been shown to interfere with DNA primer extension as well as with the formation of the RT-DNA complex. Steady-state kinetic studies demonstrate that this inhibitor can be considered as an allosteric inhibitor of HIV-1 RT. The target site on the enzyme may be also spatially related to the substrate binding site, since this inhibitor behaves competitively with respect to dTTP with poly(rA).oligo(dT) as template primer. Chemical transformations of the five phenol groups of polycitone A by methoxy groups have a determinant effect on the inhibitory potency. Thus, the pentamethoxy derivative which is devoid of all hydroxy moieties, loses significantly, by 40-fold, the ability to inhibit the DNA polymerase function. Furthermore, this analogue lacks the ability to inhibit DNA primer extension as well as the formation of the RT-DNA complex. Indeed, inhibition of the first step in DNA polymerization, the formation of the RT-DNA complex, and hence, of the overall process, could serve as a model for a universal inhibitor of the superfamily of DNA polymerases.

Inhibition of androgen action by dehydroepiandrosterone sulfotransferase transfected in PC-3 prostate cancer cells.

Age-dependent loss of androgen sensitivity of the rat liver is associated with a marked increase in dehydroepiandrosterone/hydroxysteroid sulfotransferase (rStd) activity. Sulfonated steroid hormones are known to be ineffective in binding receptor proteins. These observations suggest that intracellular androgen sulfonation can physiologically influence androgen action. We have examined the inhibitory effect of rStd on androgen action in the human prostate cancer-derived PC-3 cells transfected with the rat androgen receptor (AR) expression plasmid and two androgen-responsive promoter reporter constructs (murine mammary tumor long-terminal repeat ligated to chloramphenicol acetyltransferase (CAT) gene and rat probasin androgen response element (ARE) ligated to firefly luciferase (LUC) gene). These transfected cells were dependent on 5alpha-dihydrotestosterone (DHT) for the activation of both reporter genes and showed about a 200- and a 800-fold increase of CAT and LUC activity, respectively, at 10(-10) M DHT over the no-hormone control. Expression of the sulfonating enzyme in this cell transfection system via the rStd expression plasmid caused a dose-dependent decline in the reporter activity with approximately 90% inhibition of androgen action at a rStd:AR plasmid ratio of 100. From these results we conclude that irrespective of a high level of AR, changes in the Std expression can markedly alter the androgen sensitivity of target cells.

Reverse transcriptase of mouse mammary tumour virus: expression in bacteria, purification and biochemical characterization.

We have constructed a plasmid that induces in bacteria the synthesis of an enzymically active reverse transcriptase (RT) of mouse mammary tumour virus (MMTV), a retrovirus with a typical B-type morphology. The highest catalytic activity was detected only when 27 residues from the C-terminus of the protease were included in the N-terminus of the recombinant RT, after an extra deoxyadenosine was added between the pro and pol genes to overcome the -1 frameshift event (which occurs naturally in virus-infected cells). The recombinant protein with a six-histidine tag was purified to homogeneity by a two-column purification procedure, Ni2+ nitriloacetic acid/agarose followed by carboxymethyl-Sepharose chromatography. Unlike most RTs, the purified MMTV RT is enzymically active as a monomer even after binding a DNA substrate. Like all RTs studied, the recombinant MMTV RT possesses RNA-dependent and DNA-dependent DNA polymerase activities as well as RNase H activity, all of which show a preference for Mg2+ over Mn2+ ions. Other features of these enzymic activities, such as extension of DNA primers, processivity of DNA synthesis, pH dependence, steady-state kinetic constants, effects of Na+ or K+ ions and sensitivity to a thiol-specific reagent and to a zinc chelator, have been evaluated. The catalytic properties of MMTV RT were compared with those of the well-studied RT of HIV-1, the causative agent of AIDS. Interestingly, MMTV RT exhibits a high sensitivity to nucleoside triphosphate analogues (which are known to be potent inhibitors of HIV RTs and are being used as the major anti-AIDS drugs), as high as that of HIV-1 and HIV-2 RTs. Furthermore the recombinant MMTV RT shows a processivity of DNA synthesis higher than that of HIV-1 RT.

Kinetic properties and stereospecificity of the monomeric dUTPase from herpes simplex virus type 1.

Kinetic properties of the monomeric enzyme dUTPase from herpes simplex virus type 1 (HSV) were investigated and compared to those previously determined for homotrimeric dUTPases of bacterial and retroviral origins. The HSV and Escherichia coli dUTPases are equally potent as catalysts towards the native substrate dUTP with a kcat/K(M) of about 10(7) M(-1) s(-1) and a K(M) of 0.3 microM. However, the viral enzymes are less specific than the bacterial enzyme. The HSV and E. coli dUTPases show the same stereospecificity towards the racemic substrate analogue dUTPalphaS (2'-deoxyuridine 5'-(alpha-thio)triphosphate), suggesting that they have identical reaction mechanisms.

Oligoribonucleotides containing 8-oxo-7,8-dihydroguanosine and 8-oxo-7,8-dihydro-2'-O-methylguanosine: synthesis and base pairing properties.

7,8-Dihydroguanosine(8-oxoG) and 7,8-dihydro-2'-O-methylguanosine (8-oxoG-Me) were chemically synthesized. Oligoribonucleotides which contain 8-oxoG or 8-oxoG-Me were synthesized. Incorporation of nucleotides opposite 8-oxoG and 8-oxoG-Me with dNTP by MMLV reverse transcriptase during cDNA synthesis was characterized.

DNA synthesis exhibited by the reverse transcriptase of mouse mammary tumor virus: processivity and fidelity of misinsertion and mispair extension.

We have recently expressed in bacteria an enzymatically active reverse transcriptase (RT) of mouse mammary tumor virus (MMTV), a mammalian retrovirus with a typical B-type morphology [Taube, R., Loya, S., Avidan, O., Perach, M. & Hizi, A. (1998) Biochemical J. 329, 579-587]. The purified recombinant protein was shown to possess the catalytic activities characteristic of retroviral reverse transcriptases. In the present study, we have analyzed two basic parameters characteristic of the DNA polymerase activity of the novel MMTV RT, namely the processivity and the fidelity of DNA synthesis. Two features related to fidelity were studied, the capacity to misinsert wrong nucleotides at the 3' end of the nascent DNA strand and the ability to extend 3' mispairs. The studied properties of MMTV RT were compared with those of the RT purified from virions of avian myeloblastosis virus (AMV), since AMV RT shows a relatively high sequence similarity to MMTV RT. MMTV RT shows a relative processivity of DNA synthesis which is as high as the reference AMV RT. Regarding fidelity of DNA synthesis, MMTV RT shows a fidelity of misinsertion lower than that of AMV RT, whereas its capacity to elongate mispaired DNA is lower than that of AMV RT indicating a somewhat higher fidelity. These fidelity properties are discussed also in the context of the RTs of lentiviruses, especially those of HIV, which were reported to exhibit an exceptionally low fidelity of DNA synthesis. It is clear that MMTV RT has a fidelity higher than that of lentiviral RTs.

A human endogenous retroviral superantigen as candidate autoimmune gene in type I diabetes.

Microbial superantigens (SAGs) have been implicated in the pathogenesis of human autoimmune diseases. Preferential expansion of the Vveta7 T cell receptor positive T cell subset in patients suffering from acute-onset type I diabetes has indicated the presence of a surface membrane-bound SAG. Here, we have isolated a novel mouse mammary tumor virus-related human endogenous retrovirus. We further show that the N-terminal moiety of the envelope gene encodes an MHC class II-dependent SAG. We propose that expression of this SAG, induced in extrapancreatic and professional antigen-presenting cells, leads to beta-cell destruction via the systemic activation of autoreactive T cells. The SAG encoded by this novel retrovirus thus constitutes a candidate autoimmune gene in type I diabetes.

The dUTPases from herpes simplex virus type 1 and mouse mammary tumour virus are less specific than the Escherichia coli enzyme.

The enzyme dUTPase catalyses the hydrolysis of dUTP to dUMP and pyrophosphate, thereby suppressing incorporation of uracil into DNA and providing a pool of dUMP, the precursor of dTTP. Hydrolysis of other nucleotides similar in structure to dUTP would conceivably be physiologically detrimental and high specificity of the reaction seems to be necessary. In this work, we characterize the substrate specificity of the dUTPases from herpes simplex virus type 1 (HSV-1) and mouse mammary tumour virus (MMTV) in comparison to the Escherichia coli enzyme. We tested dCTP, dTTP, UTP and dUDP as substrates. Significantly higher reactivity was observed for the HSV-1 enzyme with dCTP and dTTP and for the MMTV enzyme with dTTP and UTP. The lower substrate specificity of the two virus enzymes compared with the bacterial enzyme is discussed in relation to the DNA precursor metabolism during virus replication.

High-affinity ssDNA inhibitors of the reverse transcriptase of type 1 human immunodeficiency virus.

The reverse transcriptase (RT) of HIV-1 is a plausible target for therapeutic agents aimed at inhibiting propagation of the virus. We have used "irrational drug design", that is, combinatorial chemistry with oligonucleotide libraries, to identify high-affinity ligands aimed at HIV-1 RT. The methodology, termed SELEX (systematic evolution of ligands by exponential enrichment), was employed with a single-stranded DNA library. The selected ssDNA ligands bind HIV-1 RT with Kd values as low as 1 nM and inhibit the RNA-dependent DNA-polymerase activity of the enzyme with Ki values as low as 0.3 nM. We also demonstrate the high specificity of one ligand able to selectively discriminate between the reverse transcriptases of HIV-1, AMV, and MMLV. These ssDNA molecules may be useful as inhibitors or as models for the design of small molecule inhibitors of HIV-1 RT in vivo.

The protein p30, encoded at the gag-pro junction of mouse mammary tumor virus, is a dUTPase fused with a nucleocapsid protein.

A ribosomal frameshift at the gag-pro junction of mouse mammary tumor virus (MMTV) gives rise to the protein p30. The protein consists of two domains, the zinc-finger-containing nucleocapsid (NC) protein portion with 95 residues and a C-terminal extension comprising 154 residues. The C-terminal domain shows similarity in sequence with the enzyme dUTPase from other sources. In this paper, we demonstrate that p30 is a functional dUTPase. Overproduction of the NC protein in Escherichia coli, using the native frameshift sequence at the gag stop codon, caused a detectable expression of dUTPase ascribed to a low frequency of readthrough. By a 1-base insertion, eliminating the gag stop codon and fusing the gag and pro reading frames, a plasmid, pET-3d-NCDU, directing overexpression of p30, was constructed. The overproduced protein, purified by phosphocellulose chromatography, shows both zinc-binding and dUTPase activity. Analytical gel filtration and sequence homology to other dUTPases suggest a trimeric assembly of p30 subunits. MMTV thus possesses two different forms of the nucleocapsid protein, the ordinary NC protein and the p30, having the NC protein connected to a domain of dUTPase.