Xenopus Rbm9 is a novel interactor of XGld2 in the cytoplasmic polyadenylation complex.

During early development, control of the poly(A) tail length by cytoplasmic polyadenylation is critical for the regulation of specific mRNA expression. Gld2, an atypical poly(A) polymerase, is involved in cytoplasmic polyadenylation in Xenopus oocytes. In this study, a new XGld2-interacting protein was identified: Xenopus RNA-binding motif protein 9 (XRbm9). This RNA-binding protein is exclusively expressed in the cytoplasm of Xenopus oocytes and interacts directly with XGld2. It is shown that XRbm9 belongs to the cytoplasmic polyadenylation complex, together with cytoplasmic polyadenylation element-binding protein (CPEB), cleavage and polyadenylation specificity factor (CPSF) and XGld2. In addition, tethered XRbm9 stimulates the translation of a reporter mRNA. The function of XGld2 in stage VI oocytes was also analysed. The injection of XGld2 antibody into oocytes inhibited polyadenylation, showing that endogenous XGld2 is required for cytoplasmic polyadenylation. Unexpectedly, XGld2 and CPEB antibody injections also led to an acceleration of meiotic maturation, suggesting that XGld2 is part of a masking complex with CPEB and is associated with repressed mRNAs in oocytes.

Revelation of a cryptic major histocompatibility complex class II-restricted tumor epitope in a novel RNA-processing enzyme.

CD4+ T-cell responses against human tumor antigens are a potentially critical component of the antitumor immune response. Molecular methods have been devised for rapidly identifying MHC class II-restricted tumor antigens and elucidating the recognized epitopes. We describe here the identification of neo-poly(A) polymerase (neo-PAP), a novel RNA processing enzyme overexpressed in a variety of human cancers, by screening a melanoma-derived invariant chain fusion cDNA library with tumor-reactive CD4+ T lymphocytes. A cryptic nonmutated HLA-DRbeta1*0701-restricted neo-PAP epitope was processed through the endogenous MHC class II pathway. A unique point mutation effected a nonconservative substitution of a leucine for a proline residue at a structurally important site in neo-PAP that was remote from the recognized peptide, revealing a normally silent epitope for immune recognition. Genetic aberrations such as the described point mutation can have unexpected immunological consequences, in this case leading to immune recognition of a distant normal self epitope.

A regulatory cytoplasmic poly(A) polymerase in Caenorhabditis elegans.

Messenger RNA regulation is a critical mode of controlling gene expression. Regulation of mRNA stability and translation is linked to controls of poly(A) tail length. Poly(A) lengthening can stabilize and translationally activate mRNAs, whereas poly(A) removal can trigger degradation and translational repression. Germline granules (for example, polar granules in flies, P granules in worms) are ribonucleoprotein particles implicated in translational control. Here we report that the Caenorhabditis elegans gene gld-2, a regulator of mitosis/meiosis decision and other germline events, encodes the catalytic moiety of a cytoplasmic poly(A) polymerase (PAP) that is associated with P granules in early embryos. Importantly, the GLD-2 protein sequence has diverged substantially from that of conventional eukaryotic PAPs, and lacks a recognizable RRM (RNA recognition motif)-like domain. GLD-2 has little PAP activity on its own, but is stimulated in vitro by GLD-3. GLD-3 is also a developmental regulator, and belongs to the Bicaudal-C family of RNA binding proteins. We suggest that GLD-2 is the prototype for a class of regulatory cytoplasmic PAPs that are recruited to specific mRNAs by a binding partner, thereby targeting those mRNAs for polyadenylation and increased expression.

Polyadenylate polymerase enzymatic activity in mammary tumor cytosols: A new independent prognostic marker in primary breast cancer.

Polyadenylate polymerase (PAP) is one of the enzymes involved in the formation of the polyadenylate tail of the 3' end of mRNA. High levels of PAP activity were associated with rapidly proliferating cells. Here we evaluate the prognostic value of PAP activity in breast cancer patients. PAP specific activity values were measured by a highly sensitive assay in the tumor cytosols of 228 women with primary breast cancer. The median follow-up period was 58 months. PAP specific activity values ranged from 2.1-39.4 units/mg protein in the breast tumor cytosols, and the activity was correlated with the level of expression of the antigen. An optimal cutoff value of 5.5 units/mg extracted protein was first defined by statistical analysis. PAP status was then compared with other established prognostic factors in terms of relapse-free survival (RFS) and overall survival (OS). PAP activity levels had a tendency to increase with tumor-node-metastasis (TNM) stage and were higher in node-positive patients. Evaluation of the prognostic value of PAP was performed using univariate and multivariate analyses. Univariate analysis showed that PAP-positive patients had a less favorable prognosis for both RFS (relative risk (RR) = 2.35; P < 0.001] and OS (RR = 3.15; P < 0.001). PAP significantly added to the prognostic power for RFS (RR = 2.51; P = 0.0012) and OS (RR = 4.21; P < 0.001) in multivariate analysis, whereas patient age, tumor size, and nodal and ER status remained independent factors for predicting survival. When only node-negative patients were examined, PAP was found to be an independent factor for predicting RFS (RR = 3.68; P = 0.0032) and OS (RR = 4.81; P < 0.001). PAP did not appear to have a prognostic significance for node-positive patients. PAP is a new prognostic factor for early recurrence and death in breast cancer patients. Our results suggest that PAP may be used as an independent unfavorable prognostic factor in node-negative breast cancer patients because there were no significant associations between PAP and the other prognostic indicators evaluated in this group of patients.

Biochemical and immunological identification and enrichment of poly(A) polymerase from human thymus.

Human thymus poly(A) polymerase (EC 2.7.7.19) activity has been investigated using poly(A) and oligo(A) as initiators. All obtained fractions reveal more than one polypeptide as detected by immunoblotting after SDS-PAGE. In addition to the homogeneously purified (Tsiapalis et al., J Biol Chem 250: 4486-4496, 1975 and Wahle, J Biol Chem 266: 3131-3139, 1991), about 60 kDa polypeptide, a larger polypeptide, about 80 kDa, that comigrates in the region of poly(A) polymerase activity was detected, enriched and partially characterized; it appears having similar size with bovine poly(A) polymerase cloned in E. coli. Polyclonal antiserum produced against recombinant bovine poly(A) polymerase reacts more efficiently with the about 80 kDa polypeptide upon immunoblotting, and can precipitate the poly(A) polymerase activity. This enzyme form, from human tissue, is novel in terms of size and may reflect intact or physiological form of poly(A) polymerase in human thymus, and supports and substantiates recent reports on the enzyme from other sources.

Monoclonal antibodies to yeast poly(A) polymerase (PAP) provide evidence for association of PAP with cleavage factor I.

Purified yeast poly(A) polymerase (PAP) was used to produce monoclonal antibodies which recognize the enzyme in immunoblots. Epitope mapping using truncated forms of PAP and cyanogen bromide cleavage products revealed two classes of antibodies. One class (N-term) recognizes an epitope in the first 100 amino acids, and a second class (C-term) is specific for a determinant located in the last 20 amino acids of PAP. These C-terminal 20 amino acids can be removed without affecting the nonspecific poly(A) addition activity of the purified enzyme. Neither antibody inhibits the nonspecific poly(A) polymerase activity or the sequence-specific activity observed in processing extracts. The antibodies show species specificity and cannot recognize mammalian, Xenopus, or vaccinia PAP. The C-term antibodies can deplete PAP from yeast whole cell extracts, resulting in loss of poly(A) addition activity. This immunodepletion also causes a reduction in the cleavage activity which can be restored by addition of yeast cleavage factor I [CF I; Chen, J., & Moore, C. (1992) Mol. Cell Biol. 12, 3470-3481], a factor needed for both the cleavage and poly(A) addition reactions. This demonstrates that a complex of PAP and CF I exists in extracts in the absence of ATP or exogenous RNA substrate. The monoclonal antibodies against yeast PAP will be a useful tool for further study of factors required for yeast mRNA 3' end processing.

Multiple forms of poly(A) polymerases in human cells.

We have cloned human poly(A) polymerase (PAP) mRNA as cDNA in Escherichia coli. The primary structure of the mRNA was determined and compared to the bovine PAP mRNA sequence. The two sequences were 97% identical at the nucleotide level, which translated into 99% similarity at the amino acid level. Polypeptides representing recombinant PAP were expressed in E. coli, purified, and used as antigens to generate monoclonal antibodies. Western blot analysis using these monoclonal antibodies as probes revealed three PAPs, having estimated molecular masses of 90, 100, and 106 kDa in HeLa cell extracts. Fractionation of HeLa cells showed that the 90-kDa polypeptide was nuclear while the 100- and 106-kDa species were present in both nuclear and cytoplasmic fractions. The 106-kDa PAP was most likely a phosphorylated derivative of the 100-kDa species. PAP activity was recovered in vitro by using purified recombinant human PAP. Subsequent mutational analysis revealed that both the N- and C-terminal regions of PAP were important for activity and suggested that cleavage and polyadenylylation specificity factor (CPSF) interacted with the C-terminal region of PAP. Interestingly, tentative phosphorylation sites have been identified in this region, suggesting that phosphorylation/dephosphorylation may regulate the interaction between the two polyadenylylation factors PAP and CPSF.

Anti-poly(A) polymerase antibodies in the sera of tumor-bearing rats can inhibit specific cleavage and polyadenylation reactions.

Sera from rats bearing Morris hepatoma 3924A for 7 weeks contained antibodies against the 48 kd tumor-type poly(A) polymerase, whereas sera from normal rats or rats bearing the tumor for shorter periods did not exhibit immunoreactivity against the enzyme. Purified IgG from the sera of the tumor-bearing rats inhibited both cleavage at the poly(A) site and polyadenylation of adenovirus L3 RNA in nuclear extracts derived from HeLa cells. By contrast, IgG from normal rats did not block the 3'-terminal processing reaction. Control or immune IgG had no effect on the transcription of ribosomal gene in the extracts derived from H-4 hepatoma cells. These data demonstrate the functional specificity of the anti-poly(A) polymerase antibodies found in the sera of the tumor-bearing rats.

Potential role of poly(A) polymerase in the assembly of polyadenylation-specific RNP complexes.

To elucidate the mechanism by which poly(A) polymerase functions in the 3'-end processing of pre-mRNAs, polyadenylation-specific RNP complexes were isolated by sedimentation in sucrose density gradients and the fractions were analyzed for the presence of the enzyme. At early stages of the reaction, the RNP complexes were resolved into distinct peaks which sedimented at approximately 18S and 25S. When reactions were carried out under conditions which support cleavage or polyadenylation, the pre-mRNA was specifically assembled into the larger 25S RNP complexes. Polyclonal antibodies raised against the enzyme purified from a rat hepatoma, which have been shown to inhibit cleavage and polyadenylation (Terns, M., and Jacob, S. T., Mol. Cell. Biol. 9:1435-1444, 1989) also prevented assembly of the 25S polyadenylation-specific RNP complexes. Furthermore, formation of these complexes required the presence of a chromatographic fraction containing poly(A) polymerase. UV cross-linking analysis indicated that the purified enzyme could be readily cross-linked to pre-mRNA but in an apparent sequence-independent manner. Reconstitution studies with the fractionated components showed that formation of the 25S RNP complex required the poly(A) polymerase fraction. Although the enzyme has not been directly localized to the specific complexes, the data presented in this report supports the role of poly(A) polymerase as an essential component of polyadenylation-specific complexes which functions both as a structural and enzymatic constituent.

Tissue and species distribution of liver type and tumor type nuclear poly(A) polymerases.

Previous studies in this laboratory have identified two distinct nuclear poly(A) polymerases, a 48 kDA tumor type enzyme and a 36-38 kDA liver type enzyme. To investigate the tissue and species specificity of these enzymes, nuclear extracts were prepared from various rat tissues, pig brain and two human cell lines. These as well as whole cell extract from yeast were probed for the two enzymes by immunoblot analysis using polyclonal anti-tumor poly(A) polymerase antibodies or autoimmune sera which contain antibodies specific for the liver type enzyme. Results indicate that both tumor and liver type enzymes are conserved across species ranging from rat to human. The yeast enzyme does not appear to be immunologically related to the liver or the tumor type poly(A) polymerase. The liver type enzyme appears to be specific for normal tissues whereas the tumor type enzyme is detected only in tissues in a "tumorigenic" state or cell lines originating from tumor tissues.

Multiple forms of poly(A) polymerases purified from HeLa cells function in specific mRNA 3'-end formation.

Poly(A) polymerases (PAPs) from HeLa cell cytoplasmic and nuclear fractions were extensively purified by using a combination of fast protein liquid chromatography and standard chromatographic methods. Several forms of the enzyme were identified, two from the nuclear fraction (NE PAPs I and II) and one from the cytoplasmic fraction (S100 PAP). NE PAP I had chromatographic properties similar to those of S100 PAP, and both enzymes displayed higher activities in the presence of Mn2+ than in the presence of Mg2+, whereas NE PAP II was chromatographically distinct and had approximately equal levels of activity in the presence of Mn2+ and Mg2+. Each of the enzymes, when mixed with other nuclear fractions containing cleavage or specificity factors, was able to reconstitute efficient cleavage and polyadenylation of pre-mRNAs containing an AAUAAA sequence element. The PAPs alone, however, showed no preference for precursors containing an intact AAUAAA sequence over a mutated one, providing further evidence that the PAPs have no intrinsic ability to recognize poly(A) addition sites. Two additional properties of the three enzymes suggest that they are related: sedimentation in glycerol density gradients indicated that the native size of each enzyme is approximately 50 to 60 kilodaltons, and antibodies against a rat hepatoma PAP inhibited the ability of each enzyme to function in AAUAAA-dependent polyadenylation.

Polyadenylate polymerases from normal and cancer cells and their potential role in messenger RNA processing: a review.

Two structurally and immunologically distinct species of nuclear polyadenylate [poly(A)] polymerases have been characterized. One of these enzymes is relatively absent in normal tissues but is predominant in primary and transplanted tumors and transformed cell lines. The presence of the tumor type enzyme in fetal liver, but not in regenerating liver, suggests that it is an oncofetal protein. Antibodies against the tumor-type poly(A) polymerases are present in the sera of rats bearing tumors and in some cancer patients. These antibodies are also found in the sera of rats fed hepatocarcinogen even before preneoplastic nodules were visible, which suggests that elicitation of these antibodies is an early event in neoplastic transformation. Autoantibodies against both liver-type and tumor-type poly(A) polymerase are also present in some rheumatic autoimmune sera. Polyclonal antibodies against purified enzyme from a rat hepatoma, which exhibit a single band upon immunoblot analysis, were used in cell-free extracts to study the role of poly(A) polymerase in the 3'-end processing of pre-mRNA. These studies showed that the antibodies blocked both endonucleolytic cleavage and poly(A) addition at the cleavage site and complex formation between factors in the extract and pre-mRNA. Independent studies in other laboratories have demonstrated that both the cleavage and poly(A) polymerase activities require the same component for their function. These observations suggest that both cleavage and polyadenylation reactions are tightly coupled in a functional complex.

Role of poly(A) polymerase in the cleavage and polyadenylation of mRNA precursor.

To determine the role of poly(A) polymerase in 3'-end processing of mRNA, the effect of purified poly(A) polymerase antibodies on endonucleolytic cleavage and polyadenylation was studied in HeLa nuclear extracts, using adenovirus L3 pre-mRNA as the substrate. Both Mg2+- and Mn2+-dependent reactions catalyzing addition of 200 to 250 and 400 to 800 adenylic acid residues, respectively, were inhibited by the antibodies, which suggested that the two reactions were catalyzed by the same enzyme. Anti-poly(A) polymerase antibodies also inhibited the cleavage reaction when the reaction was coupled or chemically uncoupled with polyadenylation. These antibodies also prevented formation of specific complexes between the RNA substrate and components of nuclear extracts during cleavage or polyadenylation, with the concurrent appearance of another, antibody-specific complex. These studies demonstrate that (i) previously characterized poly(A) polymerase is the enzyme responsible for addition of the poly(A) tract at the correct cleavage site and probably for the elongation of poly(A) chains and (ii) the coupling of these two 3'-end processing reactions appears to result from the potential requirement of poly(A) polymerase for the cleavage reaction. The results suggest that the specific endonuclease is associated with poly(A) polymerase in a functional complex.

Antibodies against nuclear poly(A) polymerases in rheumatic autoimmune diseases.

Sera from 53 patients, 26 with systemic lupus erythematosus (SLE), 8 with rheumatoid arthritis (RA), 9 with Sjogren's syndrome (SS), and 10 with scleroderma (Scl), were screened for the presence of antibodies against liver-type poly(A) polymerase and tumor-type poly(A) polymerase. Sixty percent of the patients with the above four autoimmune diseases have antibodies directed against liver poly(A) polymerase, whereas sera from 74% of the patients contained anti-hepatoma poly(A) polymerase antibodies. About 25% of the patients produced antibodies exclusively against the tumor poly(A) polymerase. IgG containing anti-liver or anti-tumor poly(A) polymerase antibodies inhibited the activity of the respective enzyme. IgG containing antibodies against liver and tumor enzymes inhibited the activity of both enzymes, whereas IgG from sera that did not react with poly(A) polymerase had no effect on either enzyme. These data demonstrated the specificity of these autoantibodies and confirmed the results of the radioimmunoassay.

Immunization of rabbits with purified RNA polymerase I induces a distinct population of antibodies against nucleic acids as well as anti-RNA polymerase I antibodies, both characteristic of systemic lupus erythematosus.

Rabbits were immunized with either RNA polymerase I or poly(A) polymerase that had been purified to apparent homogeneity and was devoid of nucleic acids. Sera from rabbits thus immunized were screened for antibodies against nucleic acids. All seven rabbits injected with RNA polymerase I but none of the four rabbits immunized with poly(A) polymerase produced anti-nucleic acid antibodies. Anti-RNA polymerase I antibodies were induced after a single injection of the enzyme. Anti-polynucleotide antibodies were not detectable until after the second immunization. Anti-RNA polymerase I antibodies could be detected with as little as 100 pg of purified RNA polymerase I in the radioimmunoassay. At least 50 ng of poly(A) or 200 ng of DNA was required to detect anti-nucleic acid antibodies. The immunoreactivity of anti-RNA polymerase I antisera was greater with synthetic polynucleotides than with DNA, particularly early in the immunization schedule. Alkaline phosphatase treatment of poly(A) to remove 5' phosphates nearly abolished its antigenicity with respect to the early sera and decreased antibody binding of later sera by 60%. These results indicate that the anti-nucleic acid antibodies produced early were primarily directed against determinants including the 5'-terminal phosphates while antibodies produced later were directed against other sites. The antinucleic acid antibodies and anti-RNA polymerase I antibodies formed two distinct populations that were not immunologically crossreactive. We suggest that after injection, RNA polymerase I becomes associated with the nucleic acids present in blood plasma which renders them immunogenic; thus, association of nucleic acids with autoimmunogenic RNA polymerase I may be one of the mechanisms by which anti-DNA antibodies are induced in systemic lupus erythematosus.

Comparison of cytosolic and nuclear poly(A) polymerases from rat liver and a hepatoma: structural and immunological properties and response to NI-type protein kinases.

Poly(A) polymerases were purified from the cytosol fraction of rat liver and Morris hepatoma 3924A and compared to previously purified nuclear poly(A) polymerases. Chromatographic fractionation of the hepatoma cytosol on a DEAE-Sephadex column yielded approximately 5 times as much poly(A) polymerase as was obtained from fractionation of the liver cytosol. Hepatoma cytosol contained a single poly(A) polymerase species [48 kilodaltons (kDa)] which was indistinguishable from the hepatoma nuclear enzyme (48 kDa) on the basis of CNBr cleavage maps. Liver cytosol contained two poly(A) polymerase species (40 and 48 kDa). The CNBr cleavage patterns of these two enzymes were distinct from each other. However, the cleavage pattern of the 40-kDa enzyme was similar to that of the major liver nuclear poly(A) polymerase (36 kDa), and approximately three-fourths of the peptide fragments derived from the 48-kDa species were identical with those from the hepatoma enzymes (48 kDa). NI-type protein kinases from liver or hepatoma stimulated hepatoma nuclear and cytosolic poly(A) polymerases 4-6-fold. In contrast, the liver cytosolic 40- and 48-kDa poly(A) polymerases were stimulated only slightly or inhibited by similar units of the protein kinases. Antibodies produced in rabbits against purified hepatoma nuclear poly(A) polymerase reacted equally well with hepatoma nuclear and cytosolic enzyme but only 80% as well with the liver cytosolic 48-kDa poly(A) polymerase and not at all with liver cytosolic 40-kDa or nuclear 36-kDa enzymes. Anti-poly(A) polymerase antibodies present in the serum of a hepatoma-bearing rat reacted with hepatoma nuclear and cytosolic poly(A) polymerases to the same extent but only 40% as well with the liver cytosolic 48-kDa enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and immunological identity of rat hepatoma and fetal liver nuclear poly(A) polymerase.

Poly(A) polymerase was partially purified from isolated nuclei of fetal rat liver. Antibodies produced in rabbits immunized with purified nuclear poly(A) polymerase from a rat hepatoma exhibited nearly identical affinity for the partially purified fetal liver and hepatoma enzymes. The extent of the antibody reaction with adult liver nuclear poly(A) polymerase partially purified in a similar manner was only 1.4% of that obtained with the hepatoma enzyme. Immune complex formation was observed between the antibodies and a major polypeptide in the fetal liver enzyme preparation which corresponded to the hepatoma enzyme (mol. wt. 48 000). No other polypeptide in the fetal liver enzyme preparation reacted with the antibodies. The 48-kDa fetal liver polypeptide produced a CNBr cleavage pattern identical to that of hepatoma poly(A) polymerase which is known to be different from the cleavage pattern of the adult liver major nuclear poly(A) polymerase. A fetal liver polypeptide corresponding to the adult liver enzyme (mol. wt. 38 000) was not evident. These results coupled with other data suggest that the hepatoma nuclear poly(A) polymerase is an oncofetal protein.

Induction of a distinct nuclear poly(A) polymerase and the production of anti-tumor poly(A) polymerase antibodies in hepatocarcinogenesis.

Primary hepatomas induced in rats by an azo dye contained a distinct nuclear poly(A) polymerase which was different from the corresponding normal liver enzyme with respect to molecular weight and peptide map. The two enzymes could also be distinguished by their immunological characteristics. Sera from the carcinogen-fed rats contained antibodies to the tumor poly(A) polymerase even before the appearance of neoplastic nodules.

Structurally and immunologically distinct poly(A) polymerases in rat liver. Occurrence of a tumor-type enzyme in normal liver.

Poly(A) polymerases purified from rat liver nuclei consisted of two distinct species, a predominant enzyme of Mr = 38,000 and a minor one of Mr = 48,000. Prior to extensive purification, the minor enzyme constituted approximately 1% of the total liver poly(A) polymerase. Poly(A) polymerase purified from a rat tumor, Morris hepatoma 3924A, was comprised of a single species of Mr = 48,000 which was identical to the minor liver enzyme with respect to chromatographic and immunological characteristics. Gel filtration on Sephacryl S-200 using 0.3 M NaCl for elution showed that the major liver poly(A) polymerase had a molecular weight of 156,000, which corresponded to a tetramer of the 38-kDa polypeptide, whereas the hepatoma and minor liver 48-kDa species existed as dimers with a molecular weight of 96,000. Fractionation by Sephacryl S-200 resulted in complete loss of both liver poly(A) polymerase activities which could be restored by exogenous N1-type protein kinase. Following CNBr cleavage, the 48-kDa poly(A) polymerase from liver and hepatoma exhibited nearly identical peptide maps which were distinct from that of the major liver enzyme (38 kDa). Antibodies raised against tumor poly(A) polymerase reacted with the 48-kDa polypeptide but not with the 38-kDa liver enzyme. Immune complex formation was observed between seven of the eight CNBr cleavage products derived from the 48-kDa polypeptide of both liver and hepatoma. It is concluded that distinct genes in rat liver code for two structurally and immunologically unique nuclear poly(A) polymerases, one of which is identical to the enzyme from the hepatoma.

Phosphorylation and immunology of poly(A) polymerase.

Phosphorylation of poly(A) polymerase by protein kinase NI (a cyclic nucleotide-independent nuclear kinase closely associated with poly(A) polymerase at early stages of purification) resulted in as much as 7-fold activation of poly(A) polymerase. Phosphorylation causes an increase in the rate rather than the extent of polyadenylation. Antibodies raised in rabbits against purified poly(A) polymerase from Morris hepatoma 3924A reacted specifically with poly(A) polymerase following "Western" transfer of the enzyme onto diazobenzyloxyl methyl paper. Using iodinated enzyme, a competition radioimmunoassay for poly(A) polymerase was developed. Using the radioimmunoassay, it was shown that Morris hepatoma 3924A contains 100 micrograms of poly(A) polymerase/mg DNA or 10(7) molecules of the enzyme/cell nucleus. Nuclear poly(A) polymerase from fetal liver, but not from normal liver, was able to compete well with hepatoma enzyme in the radioimmunoassay. These data suggest that the tumor poly(A) polymerase is probably an oncofetal antigen, resulting from derepression of a gene not normally expressed in adult liver.