Identification of specific adenovirus E1A N-terminal residues critical to the binding of cellular proteins and to the control of cell growth

Repression of the c-Jun trans-activation function by the adenovirus type 12 E1A 52R protein correlates with the inhibition of phosphorylation of the c-Jun activation domain

The early region 1A 52R polypeptide, a protein expressed exclusively by the in vivo oncogenic adenovirus subtype 12, represses the trans-activating function of the cellular transcription factor complex AP-1 consisting of c-Jun-c-Jun homodimers. In this report we demonstrate that the repression in vivo correlates with a direct physical interaction of the adenovirus protein with c-Jun in vitro. Interestingly, the 52R protein binds to the bZIP domain of c-Jun essential for dimerization and DNA binding but not to the c-Jun activation domain. This interaction does not prevent the promoter binding of c-Jun/AP-1. Moreover, the physical association between c-Jun and the TATA box-binding protein TBP is not disturbed by the 52R polypeptide. In fact, we show evidence that down-regulation of c-Jun activity by the adenoviral protein is due to the inhibition of phosphorylation of the c-Jun trans-activation domain. In vivo phosphorylation of the c-Jun activation domain is necessary for the interaction of c-Jun with specific cofactors such as CBP and therefore a prerequisite for the activation of target genes. Due to these results we propose a model in which the 52R protein represses the trans-activating function of c-Jun by preventing its phosphorylation through a specific kinase necessary for the activation of the cellular transcription factor.

Repression in vitro, by human adenovirus E1A protein domains, of basal or Tat-activated transcription of the human immunodeficiency virus type 1 long terminal repeat

Human adenovirus E1A proteins can repress the expression of several viral and cellular genes. By using a cell-free transcription system, we demonstrated that the gene product of the E1A 12S mRNA, the 243-residue protein E1A243R, inhibits basal transcription from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). The HIV-1 transactivator protein Tat greatly stimulates transcription from the viral promoter in vitro. However, E1A243R can repress Tat-activated transcription in vitro. Strong repression of both basal and Tat-activated transcriptions requires only E1A N-terminal amino acid residues 1 to 80. Deletion analysis showed that E1A N-terminal amino acids 4 to 25 are essential for repression, whereas amino acid residues 30 to 49 and 70 to 80 are dispensable. Transcriptional repression by E1A in the cell-free transcription system is promoter specific, since under identical conditions, transcription of the adenovirus major late promoter and the Rous sarcoma virus LTR promoter was unaffected. The repression of transcription by small E1A peptides in vitro provides an assay for investigation of molecular mechanisms governing E1A-mediated repression of both basal and Tat-activated transcriptions of the HIV-1 LTR promoter.

Induction by adenovirus-5 E1A of the differentiation phenotype of F9 teratocarcinoma cells involves a conserved region (CR1) of E1A

The effects of the E1A protein of adenovirus-5 on the differentiation program of F9 teratocarcinoma cells were examined by the stable introduction of plasmids that expressed wild-type or mutated forms of E1A. Constitutive expression of plasmids for most of the mutant E1As induced loss of expression of the cell-surface antigen SSEA-1 and the enhanced expression of genes specific for the differentiated phenotype of F9 cells, such as genes for laminin B1, tissue-type plasminogen activator (tPA) and type IV collagen, as well as the altered cell morphology that is associated with the differentiated state. However, such changes were not observed in the case of genes for mutant proteins from which a conserved region (CR1) of E1A had been deleted. Furthermore, no significant induction of expression of the c-jun gene or transactivation of the c-jun-CAT reporter gene were observed when the sequence that encodes CR1 of E1A had been deleted. A palindromic sequence element (DRE) of the c-jun promoter was essential for the E1A-mediated up-regulation of the c-jun gene. These results imply that CR1 is required for activation of the c-jun gene and that it is implicated in the growth arrest, expression of parietal endoderm-specific functions and the orderly differentiation of F9 cells.

Adenovirus E1A represses cardiac gene transcription and reactivates DNA synthesis in ventricular myocytes, via alternative pocket protein- and p300-binding domains

To examine the potential impact of disrupting "pocket" protein function on cardiac differentiation and growth, we introduced 12 S E1A genes into neonatal ventricular myocytes, by adenoviral gene transfer. In the absence of E1B, E1A was cytotoxic, with features typical of apoptosis. In the presence of E1B, E1A preferentially inhibited transcription of cardiac-restricted alpha-actin promoters, and reactivated DNA synthesis in cardiac myocytes, without cell death. Mutations that abrogate known activities of the amino terminus of E1A, versus conserved region 2, demonstrate that the "pocket" protein- and p300-binding domains each suffice, in the absence of the other, for transcriptional repression and re-entry into S phase.

Adenoviral E1A-associated protein p300 as a functional homologue of the transcriptional co-activator CBP

The 265K nuclear protein CBP was initially identified as a co-activator for the protein kinase A (PKA)-phosphorylated form of the transcription factor CREB. The domains in CBP that are involved in CREB binding and transcriptional activation are highly related to the adenoviral E1A-associated cellular protein p300 (refs 2, 3), and to two hypothetical proteins from Caenorhabditis elegans, R10E11.1 and K03H1.10 (refs 4 and 5, respectively), whose functions are unknown. Here, we show that CBP and p300 have similar binding affinity for the PKA-phosphorylated form of CREB, and that p300 can substitute for CBP in potentiating CREB-activated gene expression. We find that E1A binds to CBP through a domain conserved with p300 and represses the CREB-dependent co-activator functions of both CBP and p300. Our results indicate that the gene repression and cell immortalization functions associated with E1A involve the inactivation of a family of related proteins that normally participate in second-messenger-regulated gene expression.

Adenovirus E1A proteins interact with the cellular YY1 transcription factor

The adenovirus 12S and 13S E1A proteins have been shown to relieve repression mediated by the cellular transcription factor YY1. The 13S E1A protein not only relieves repression but also activates transcription through YY1 binding sites. In this study, using a variety of in vivo and in vitro assays, we demonstrate that both E1A proteins can bind to YY1, although the 13S E1A protein binds more efficiently than the 12S E1A protein. Two domains on the E1A proteins interact with YY1: an amino-terminal sequence (residues 15 to 35) that is present in both E1A proteins and a domain that includes at least a portion of conserved region 3 (residues 140 to 188) that is present in the 13S but not the 12S E1A protein. Two domains on YY1 interact with E1A proteins: one is contained within residues 54 to 260, and the other is contained within the carboxy-terminal domain of YY1 (residues 332 to 414). Cotransfection of a plasmid expressing carboxy-terminal amino acids 332 to 414 of YY1 fused to the GAL4 DNA-binding domain can inhibit expression from a reporter construct with GAL4 DNA binding sites in its promoter, and inclusion of a third plasmid expressing E1A proteins can relieve the repression. Thus, we find a correlation between the ability of E1A to interact with the carboxy-terminal domain of YY1 and its ability to relieve repression caused by the carboxy-terminal domain of YY1. We propose that E1A proteins normally relieve YY1-mediated transcriptional repression by binding directly to the cellular transcription factor.

Mitotic cycle reactivation in terminally differentiated cells by adenovirus infection

Different cell types (e.g., neurons, skeletal and heart myocytes, adipocytes, keratinocytes) undergo terminal differentiation, in which acquisition of specialized functions entails definitive withdrawal from the cell cycle. Such cells are distinct from quiescent (reversibly growth-arrested) cells, such as contact-inhibited fibroblasts. Terminally differentiated cells can not be induced to proliferate by means of growth factor stimulation or transduction of cellular oncogenes. An important first step toward defining the molecular basis for such unresponsiveness is to find a practical means to overcome the proliferative block. Furthermore, determining whether terminally differentiated, postmitotic cells still retain a potential competence for proliferation that can be reactivated would have important theoretical and practical implications. To address these questions, we exploited the properties of adenoviruses. These viruses can infect postmitotic cells and express E1A, a powerful activator of proliferation in reversibly growth-arrested cells. We infected terminally differentiated skeletal muscle cells and adipocytes with human adenovirus type 5 or 12, obtaining full reentry into the cell cycle, including DNA synthesis, mitosis, cytokinesis, and extended proliferation. Similar results were obtained with established cell lines and primary cells belonging to several species, from quail to humans. Genetic analysis indicated that the smaller splice product of E1A, E1A 12S, is sufficient to induce cell cycle reactivation in otherwise permanently nonmitotic cells. These results demonstrate that terminally differentiated cells retain proliferative potential and establish adenovirus as a convenient and powerful means to force such cells to reenter the cell cycle.

Protein domains connect cell cycle stimulation directly to initiation of DNA replication

Polyoma large T antigen (LT) is the only viral gene product required for viral DNA replication. LT can be divided into two domains, one N-terminal (NT) spanning residues 1-260 and one C-terminal (CT) comprising approximately residues 264-785. NT is known to immortalize primary cells in a manner dependent on binding of pRB/p107. Here a CT construct comprising residues 264-785 was shown to have independent function in DNA replication. CT is entirely sufficient for driving viral DNA replication in vivo in growing mouse cells at a level approaching that of full-length LT. In contrast, CT is strikingly deficient for replication in serum-starved cells. However, this deficiency can be complemented by coexpression of NT. BrdUrd incorporation in transfected, starved cells showed that NT was sufficient for inducing S phase, suggesting a mechanism for complementation. By contrast, CT was unable to induce S phase when tested in the same assay. NT also promotes phosphorylation of sites in CT that are likely to be important for replication. Other DNA tumor virus gene products such as adenovirus E1A 12S and human papillomavirus 16 E7 could also complement CT for replication. Although NT, E1A 12S, and E7 all bind the retinoblastoma gene product (pRB) and p107, genetic analysis demonstrates an additional function, independent of that binding, is responsible for complementation.

Differential specificity for binding of retinoblastoma binding protein 2 to RB, p107, and TATA-binding protein

The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p107, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/E1A domains and with TBP only through the non-T/E1A domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB- and TBP-binding activities were retained. We located the non-T/E1A binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/E1A binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.

Differential specificity for binding of retinoblastoma binding protein 2 to RB, p107, and TATA-binding protein

The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p107, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/E1A domains and with TBP only through the non-T/E1A domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB- and TBP-binding activities were retained. We located the non-T/E1A binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/E1A binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.

Transrepression of RNA polymerase II promoters by the simian virus 40 small t antigen

Simian virus 40 (SV40) small t antigen (t) can activate transcription from certain RNA polymerase II and III promoters (M. Loeken, I. Bikel, D. M. Livingston, and J. Brady, Cell 55:1171-1177, 1988). Here we report a new function of t, its ability to repress human c-fos promoter and AP-1 transcriptional activity in CV-1P cells. This function is the product of a discrete N-terminal domain of t, because the large T antigen (T)/t-common polypeptide, which contains only the first 82 amino acids common to both T and t of SV40, was, like the intact protein, an active repressor. The data further suggest that the t- and T/t-common-mediated repression of c-fos expression was most likely manifest at the level of transcription. In keeping with the possibility that t affects the expression of the genomic c-fos promoter, it also led to repression of AP-1 formation. Thus, SV40 is both an activator and a repressor of transcription. Its ability to inhibit c-fos expression should be considered in light of the natural history of SV40 in its natural host.

Functional importance of complex formation between the retinoblastoma tumor suppressor family and adenovirus E1A proteins as determined by mutational analysis of E1A conserved region 2

Adenovirus early region 1A (E1A) products induce DNA synthesis, transform primary rodent cells, and activate transcription factor E2F through complex formation with an array of cellular proteins via the E1A amino terminus and conserved regions 1 and 2 (CR1 and CR2). Interactions with the retinoblastoma tumor suppressor, pRb, and related proteins p107 and p130 rely somewhat on CR1 but largely on CR2, which contains a core binding sequence Leu-122-X-Cys-X-Glu. We introduced point mutations in CR2 to define such interactions more precisely. In human cells, alteration of any of the conserved residues within the binding core eliminated complex formation with pRb. Conversion of nonconserved Thr-123 to Pro (but not to either Ala or Ser) disrupted binding of pRb, presumably because of conformational changes in the binding core. No single E1A point mutant was completely defective in binding p107, suggesting that molecular interactions between E1A proteins and p107 clearly differ from those with pRb and p130. In general, the patterns of complex formation by E1A mutants in rat, monkey, and human cells were quite similar. All mutants which failed to bind significant amounts of pRb also failed to transform primary rat cells. Several mutants demonstrated selective binding to pRb, p107, and p130, but transforming activity corresponded largely with complex formation with pRb, regardless of the levels of interactions with p107 and p130. Mutants defective for binding of both pRb and p107 failed to induce the activity of transcription factor E2F; however, quite high levels were activated by E1A mutants that interacted with p107 alone. These results suggested that both pRb and p107 are important regulators of E2F activity but that complex formation with and activation of E2F by p107 are insufficient for cell transformation.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.

An adenovirus E1A transcriptional repressor domain functions as an activator when tethered to a promoter

The adenovirus E1A protein contains three well conserved regions, designated conserved region (CR) 1, 2 and 3, which are important for the multiple activities ascribed to E1A. The CR3 domain constitutes a prototypic transcription activator, consisting of a promoter targeting region and a transactivating region. Here we demonstrate the existence of a second transactivating region located within amino acids 28 to 90 (essentially the CR1 domain) of the E1A protein. A fusion protein, containing the Gal4 DNA binding domain linked to CR1, was as efficient as the classical CR3 transactivator in activating transcription from a reporter plasmid containing Gal4 binding sites. However, competition experiments suggest that Gal/CR1 and Gal/CR3 work through different cellular targets. The E1A-243R protein has previously been extensively characterized as a repressor of transcription. Here we show that a Gal4 fusion protein expressing the CR1 domain is indeed sufficient for repression of SV40 enhancer activity. Collectively, our results suggest that CR1 functions as an activator if tethered to a promoter and as a repressor in the absence of promoter association.

Complementary functions of E1a conserved region 1 cooperate with conserved region 3 to activate adenovirus serotype 5 early promoters

The amino-terminal region of the adenovirus type 5 E1a protein including conserved regions (CRs) 1 and 2 binds the 105-kDa retinoblastoma protein and a second, 300-kDa, cellular protein. We show that mutant viruses with deletions of CR1 which release the binding of either p105 or p300 still activate early promoters and infect cells productively. However, mutations which disrupt binding of both proteins disrupt early promoter activity and block the viral life cycle. Ela CR3, which has an established role in early promoter activation, can act in trans to the amino-terminal functions. This suggests that the amino terminus provides distinct, redundant functions related to p300 and Rb binding that synergize with CR3 to transactivate early genes.

Molecular cloning and functional analysis of the adenovirus E1A-associated 300-kD protein (p300) reveals a protein with properties of a transcriptional adaptor

The growth-controlling functions of the adenovirus E1A oncoprotein depend on its ability ot interact with a set of cellular proteins. Among these are the retinoblastoma protein, p107, p130, and p300. We have isolated a cDNA encoding full-length human p300 and mapped the chromosomal location of the gene to chromosome 22q13. p300 contains three cysteine- and histidine-rich regions of which the most carboxy-terminal region interacts specifically with E1A. In its center, p300 contains a bromodomain, a hallmark of certain transcriptional coactivators. We have examined the ability of p300 to overcome the repressive effect of E1A on the SV40 enhancer. We show that p300 molecules lacking an intact E1A-binding site can bypass E1A repression and restore to a significant extent the activity of the SV40 enhancer, even in the presence of high levels of E1A protein. These results imply that p300 may function as a transcriptional adaptor protein for certain complex transcriptional regulatory elements.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

Tumorigenicity of adenovirus-transformed rodent cells is influenced by at least two regions of adenovirus type 12 early region 1A

Chimeric adenovirus type 5 (Ad5)/Ad12 early region 1A (E1A) genes were used to transform primary baby rat kidney cells in cooperation with Ad12 E1B, and the resulting cell lines were assayed for tumorigenicity in syngeneic rats. It was found that lines were nontumorigenic when transformed by hybrid E1A genes consisting of the amino-terminal 80 amino acids from Ad12 including conserved region 1 (CR1), with the remaining portion from Ad5. In contrast, cell lines transformed by hybrids containing Ad12 E1A sequences from the amino terminus to the leftmost border of CR3 or beyond were tumorigenic. To extend these results, sequences spanning CR2 and CR3 of Ad5 E1A were replaced with the homologous regions of Ad12 E1A and additional transformed cell lines were established. These lines were weakly-to-moderately tumorigenic, suggesting that Ad12 E1A sequences between CR2 and CR3 may be involved in tumorigenicity but are not the sole factors influencing it. Interestingly, examination of an E1A sequence alignment indicated that the region between CR2 and CR3 of Ad12 E1A is also conserved in the corresponding sequence of simian adenovirus type 7, which, like Ad12, is highly oncogenic. This region is characterized by the presence of a stretch of several alanine residues and is similar to a motif present in a number of proteins with transcriptional repression activity. The possibility that this region may influence tumorigenicity by means of a transcriptional regulatory mechanism is discussed.

Human cancer cell lines express a negative transcriptional regulator of the interferon regulatory factor family of DNA binding proteins

Members of the interferon regulatory factor (IRF) family of DNA binding transcription factors have roles in growth regulation, antiviral responses, and transcriptional induction of interferon (IFN)-activated early response genes. The IRF family member ISGF3 gamma is the DNA binding component of IFN-stimulated gene factor 3 (ISGF3), a multicomponent complex responsible for the stimulation of IFN-alpha-responsive genes. IFN-alpha-stimulated formation of ISGF3 and subsequent gene expression can be inhibited by phorbol esters or expression of the adenovirus E1A protein. We have investigated IFN signaling in human malignant tumor cell lines of the lung, colon, ovary, cervix, and hematopoietic organs and found some of these cells to be defective for IFN-alpha-induced formation of ISGF3. In many cases, an inhibitory activity termed transcriptional knockout (TKO) correlated with nonresponsiveness. TKO purified from a human papillomavirus-negative cervical carcinoma cell line has a molecular size of 19 kDa. The purified protein interacted with the ISGF3 gamma component of ISGF3, preventing binding of ISGF3 to DNA. Purified TKO displaced ISGF3 from its DNA binding site in vitro and prevented ISGF3 gamma, IRF-1, and IRF-2 from interacting with the IFN-stimulated response element. Partially purified TKO can also directly interact with ISGF3 gamma in the absence of DNA. This protein may be involved with the development of malignancies and the inability of IFN to exert its antiproliferative and antiviral effects.

The E1A products of oncogenic adenovirus serotype 12 include amino-terminally modified forms able to bind the retinoblastoma protein but not p300

The cell growth-regulating properties of the adenovirus type 5 (Ad5) E1A oncogene correlate closely with the binding of the E1A products to specific cellular proteins. These proteins include the products of the retinoblastoma tumor susceptibility gene and a 300-kDa product, p300. pRB binds to E1A sequences that are highly conserved among the E1A products of various serotypes, while p300 binding requires sequences in the E1A amino terminus, a region that is not highly conserved. To help evaluate the roles of the E1A-associated proteins in cell growth control, we have compared the p300-binding abilities of the E1A products of Ad5 and of the more oncogenic Ad12 serotype. We show here that despite encoding a sequence that varies somewhat from the p300-binding sequences of Ad5 E1A, the Ad12 E1A products associate with p300 with an affinity similar to that of the Ad5 E1A products. Both the 12S and 13S splice products of Ad12 E1A, like those of Ad5 E1A, encode proteins able to associate with p300. Interestingly, though, both also give rise to prominent forms that are amino terminally modified and unable to associate with p300. This modification, at least in the 13S product, does not appear to diminish the affinity of this product for the retinoblastoma protein.

The adenovirus E1A 12S product displays functional redundancy in activating the human proliferating cell nuclear antigen promoter

The adenovirus E1A 243R oncoprotein stimulates expression from the promoter of the human proliferating cell nuclear antigen (PCNA). To gain insight into the mechanism of activation, we analyzed deletion and point mutations of the 243R protein for their abilities to activate PCNA promoter-directed reporter gene expression upon cotransfection into HeLa cells. Large deletions that in combination span the entire protein severely impaired the ability of E1A 243R to induce PCNA expression. Smaller deletions and specific point mutations that target specific E1A-binding proteins were less deleterious to PCNA induction. The data suggest that E1A activates transcription of the PCNA gene by multiple mechanisms and that, of the known 243R-associated proteins, p300 and p107-cyclin A can mediate the response while p105-RB does not appear to participate. Presumably, the functional redundancy ensures that 243R can activate expression of this essential DNA replication protein regardless of cell type and physiological conditions.