Human gene for proliferating cell nuclear antigen has pseudogenes and localizes to chromosome 20

DNA Base Excision Repair in Plants: An Unfolding Story With Familiar and Novel Characters

Base excision repair (BER) is a critical genome defense pathway that deals with a broad range of non-voluminous DNA lesions induced by endogenous or exogenous genotoxic agents. BER is a complex process initiated by the excision of the damaged base, proceeds through a sequence of reactions that generate various DNA intermediates, and culminates with restoration of the original DNA structure. BER has been extensively studied in microbial and animal systems, but knowledge in plants has lagged behind until recently. Results obtained so far indicate that plants share many BER factors with other organisms, but also possess some unique features and combinations. Plant BER plays an important role in preserving genome integrity through removal of damaged bases. However, it performs additional important functions, such as the replacement of the naturally modified base 5-methylcytosine with cytosine in a plant-specific pathway for active DNA demethylation.

The PCNA pseudogenes in the human genome

BACKGROUND

The proliferating cell nuclear antigen (PCNA) is a key protein in the eukaryotic DNA replication and cell proliferation. Following the cloning and characterisation of the human PCNA gene, the question of the existence of pseudogenes in the human genome was raised.

FINDINGS

In this short communication we summarise the existing information about the PCNA pseudogenes and critically assess their status.

CONCLUSIONS

We propose the existence of at least four valid PCNA pseudogenes, PCNAP1, PCNAP2, LOC392454 and LOC390102. We would like to recommend assignment of a name for LOC392454 as "proliferating cell nuclear antigen pseudogene 3" (alias PCNAP3) and a name for LOC390102 as "proliferating cell nuclear antigen pseudogene 4" (alias PCNAP4). We prompt for more critical evaluation of the existence of a PCNA pseudogene, designated as PCNAP.

Proliferating cell nuclear antigen (PCNA): a key factor in DNA replication and cell cycle regulation

BACKGROUND

PCNA (proliferating cell nuclear antigen) has been found in the nuclei of yeast, plant and animal cells that undergo cell division, suggesting a function in cell cycle regulation and/or DNA replication. It subsequently became clear that PCNA also played a role in other processes involving the cell genome.

SCOPE

This review discusses eukaryotic PCNA, with an emphasis on plant PCNA, in terms of the protein structure and its biochemical properties as well as gene structure, organization, expression and function. PCNA exerts a tripartite function by operating as (1) a sliding clamp during DNA synthesis, (2) a polymerase switch factor and (3) a recruitment factor. Most of its functions are mediated by its interactions with various proteins involved in DNA synthesis, repair and recombination as well as in regulation of the cell cycle and chromatid cohesion. Moreover, post-translational modifications of PCNA play a key role in regulation of its functions. Finally, a phylogenetic comparison of PCNA genes suggests that the multi-functionality observed in most species is a product of evolution.

CONCLUSIONS

Most plant PCNAs exhibit features similar to those found for PCNAs of other eukaryotes. Similarities include: (1) a trimeric ring structure of the PCNA sliding clamp, (2) the involvement of PCNA in DNA replication and repair, (3) the ability to stimulate the activity of DNA polymerase δ and (4) the ability to interact with p21, a regulator of the cell cycle. However, many plant genomes seem to contain the second, probably functional, copy of the PCNA gene, in contrast to PCNA pseudogenes that are found in mammalian genomes.

Identification and functional analysis of PCNA1 and PCNA-like1 genes of Phaseolus coccineus

Proliferating cell nuclear antigen (PCNA) is an essential factor in DNA replication and in many other processes in eukaryotic cells. Genetic analysis of Phaseolus coccineus showed the presence of at least two PCNA-like genes in the runner bean genome. Two PCNA genes have previously been found in a few plant species including Arabidopsis, tobacco, and maize. In these species, genes were nearly identical. Two cDNAs of P. coccineus PCNA (PcPCNA1 and PcPCNA-like1) have been identified that differ distinctly from each other. Interestingly, both the genetic organization of PcPCNA1 and PcPCNA-like1 genes and their expression patterns were similar, but these were the only similarities between these genes and their products. The identity between PcPCNA1 and PcPCNA-like1 at the amino acid level was only 54%, with PcPCNA-like1 lacking motifs that are crucial for the activity typical of PCNA. Consequently, these two proteins showed different properties. PcPCNA1 behaved like a typical PCNA protein: it formed a homotrimer and stimulated the activity of human DNA polymerase delta. In addition, PcPCNA1 interacted with a p21 peptide and was recognized by an anti-human PCNA monoclonal antibody PC10. By contrast, PcPCNA-like1 was detected as a monomer and was unable to stimulate the DNA polymerase delta activity. PcPCNA-like1 also could not interact with p21 and was not recognized by the PC10 antibody. Our results suggest that PcPCNA-like1 either is unable to function alone and therefore might be a component of the heterotrimeric PCNA ring or may have other, yet unknown functions. Alternatively, the PcPCNA-like1 gene may represent a pseudogene.

Novel perspective: focusing on the X chromosome in reproductive cancers

In an XX female, one of the two X chromosomes has been inactivated during early embryonic life to achieve a compensation of X-linked gene products between males and females, leaving only one allele of X-linked genes functional. There are some X-linked genes escaping the X-inactivation, i.e., being expressed from both alleles. Escape from X-inactivation varies at different levels; some genes have both alleles active in some women but only one allele active in others, whereas some other genes have both alleles active in neoplastic tissue but only one allele active normally. The X-inactivation may be considered functionally equivalent to a loss of heterozygosity (LOH) for some genes, whereas escape from X-inactivation may be equivalent to functional gene amplification for others. The physiological LOH may make X-linked tumor suppressor genes lose their function more easily, compared with autosomal tumor suppressor genes, thus predisposing women to cancer formation more easily. Moreover, the human X chromosome contains many genes related to cancer or to sex and reproduction. All these properties of the X chromosome suggest that it may play more important roles than any autosomal chromosome in the development and progression of reproductive and urologic cancers.

Multiple roles of the proliferating cell nuclear antigen: DNA replication, repair and cell cycle control

The proliferating cell nuclear antigen (PCNA), the auxiliary protein of DNA polymerase delta and epsilon, is involved in DNA replication and repair. This protein forms a homotrimeric structure which, encircling DNA, loads the polymerase on the DNA template. A role for PCNA in the cell cycle control is recognised on the basis of the interaction with cyclins, cyclin-dependent kinases (cdks) and the cdk-inhibitor p21 waf1/cip1/sdi1 protein. Association with the growth-arrest and DNA-damage inducible proteins gadd45 and MyD118, further demonstrates the role of PCNA as a component of the cell cycle control apparatus.

Cloning, sequencing, and chromosomal localization of two tandemly arranged human pseudogenes for the proliferating cell nuclear antigen (PCNA)

We have characterized a human genomic clone carrying two pseudogenes for the proliferating cell nuclear antigen (PCNA), which were tandemly arranged on human Chromosome (Chr) 4. One is a processed pseudogene that showed a 73% nucleotide homology to the human PCNA cDNA and possessed none of the introns existing in the functional PCNA gene. This pseudogene presumably arose by reverse transcription of a PCNA mRNA followed by integration of the cDNA into the genome. The other is a 5' and 3' truncated pseudogene that showed a nucleotide homology to a 3' region of the exon 4 and to a 5' region of the exon 5 of the PCNA gene and did not have the intronic sequence between the exons 4 and 5. Both pseudogenes had the same nucleotide deletion as compared with the human functional PCNA gene. A phylogenetic analysis of PCNA gene family, including the functional PCNA gene and another PCNA pseudogene located on a different chromosome, revealed that the truncated pseudogene exhibits the closest evolutionary relationship with the processed pseudogene, suggesting that the truncated pseudogene was generated by duplication of the processed pseudogene after translocation to Chr 4. Furthermore, fluorescence in situ hybridization revealed that these pseudogenes are located on the long arm of Chr 4, 4q24.

Promoter activity of the proliferating-cell nuclear antigen gene is associated with inducible CRE-binding proteins in interleukin 2-stimulated T lymphocytes

The proliferating-cell nuclear antigen (PCNA) gene encodes an auxiliary factor of DNA polymerase delta and functions in DNA replication during S phase. It is expressed at much higher levels in proliferating cells than in quiescent cells. We have studied the regulatory role of the 5'-flanking sequence of the murine PCNA gene in interleukin 2 (IL-2)-responsive cloned T cells (L2). Analysis of a set of deletion constructs in transient transfection assays measuring heterologous reporter gene (luciferase) activity demonstrated that the 182-bp 5'-flanking region provides full promoter activity in IL-2-stimulated L2 cells. While many elements contribute to PCNA promoter strength in IL-2-stimulated cells, the largest decrease in activity occurred with deletion of the tandem CRE (cyclic AMP response element) binding sites located at nucleotides -37 to -52. With a gel mobility shift assay, several IL-2-inducible DNA-protein complexes were detected, including CREB (CRE-binding) and ATF1 (activating transcription factor) proteins that are specific for the PCNA-CRE sequence. Methylation interference analysis confirmed specific binding of these proteins to the CRE sites. Mutation at the PCNA-CRE motif abolishes IL-2-inducible binding and reduces substantially PCNA promoter activity. These results indicate that IL-2-stimulated PCNA transcription may be partially mediated by these CRE-binding proteins.

Promoter activity of the proliferating-cell nuclear antigen gene is associated with inducible CRE-binding proteins in interleukin 2-stimulated T lymphocytes

The proliferating-cell nuclear antigen (PCNA) gene encodes an auxiliary factor of DNA polymerase delta and functions in DNA replication during S phase. It is expressed at much higher levels in proliferating cells than in quiescent cells. We have studied the regulatory role of the 5'-flanking sequence of the murine PCNA gene in interleukin 2 (IL-2)-responsive cloned T cells (L2). Analysis of a set of deletion constructs in transient transfection assays measuring heterologous reporter gene (luciferase) activity demonstrated that the 182-bp 5'-flanking region provides full promoter activity in IL-2-stimulated L2 cells. While many elements contribute to PCNA promoter strength in IL-2-stimulated cells, the largest decrease in activity occurred with deletion of the tandem CRE (cyclic AMP response element) binding sites located at nucleotides -37 to -52. With a gel mobility shift assay, several IL-2-inducible DNA-protein complexes were detected, including CREB (CRE-binding) and ATF1 (activating transcription factor) proteins that are specific for the PCNA-CRE sequence. Methylation interference analysis confirmed specific binding of these proteins to the CRE sites. Mutation at the PCNA-CRE motif abolishes IL-2-inducible binding and reduces substantially PCNA promoter activity. These results indicate that IL-2-stimulated PCNA transcription may be partially mediated by these CRE-binding proteins.

Proliferating cell nuclear antigen (PCNA) expression in chronic lymphocytic leukemia (CLL)

Chronic Lymphocytic Leukemia (CLL) is usually an indolent disorder which in some patients assumes an aggressive clinical course. In order to assess at presentation the prognosis of a given patient, several staging systems and prognostic variables have been proposed including the expression of the Proliferating Cell Nuclear Antigen (PCNA). PCNA is a 36 kd nuclear protein, the regulation of which is cell cycle-dependent. In CLL, PCNA levels correlate with cell proliferation, clinical stage and the lymphocyte doubling time (LDT). Furthermore, preliminary data suggests that PCNA expression may also predict response to Fludarabine-based chemotherapy. Since PCNA is a cofactor for Delta DNA polymerase, PCNA overexpression in CLL may also reflect the intrinsic DNA repair activity of the leukemic cells and thus their resistance to chemotherapy. Further studies aiming at modulation of PCNA expression in CLL cells may clarify this issue and may offer a future new therapeutic strategy with which to treat this disorder.

Toxicological and pathological applications of proliferating cell nuclear antigen (PCNA), a novel endogenous marker for cell proliferation

A major stimulus to study cell proliferation, particularly in rodent carcinogenicity assays and human tumors, has been the belief that the quantification of this fundamental biological process will provide the toxicologist and pathologist with objective data allowing a better understanding of the mechanisms involved in the toxicity and/or carcinogenicity of certain compounds as well as guiding more effective management of patients afflicted with neoplasia. Among the markers used for cell proliferation measurement, PCNA has recently gained much attention and holds much promise as it is intricately involved in the cell replication processes. It not only could allow measurement of the replication rates without necessitating pretreatment of the animal/tissue in prospective studies, but also would allow retrospective assessment of the proliferative rates in archival tissues due to the conservation of this marker in fixed and paraffin-embedded tissues. Finally, knowledge of the function of PCNA in the cell cycle and its regulation by other factors may help us understand the advantages and limitations of PCNA as a cell proliferation marker in its application in toxicology and as a prognostic marker in human tumors.

Analysis of carrot genes for proliferating cell nuclear antigen homologs with the aid of the polymerase chain reaction

We designed a polymerase chain reaction-based strategy to obtain information about the origin and distribution of a newly discovered proliferating cell nuclear antigen (PCNA) homolog. Carrot genomic segments were amplified using degenerate primers for two conserved regions of known PCNA homologs. The genes encoding the larger PCNA as well as typical PCNA contained introns. Thus, unlike processed PCNA pseudogenes in mammals, the larger homolog is not generated through reverse transcription of a typical PCNA mRNA. Moreover, introns of the larger PCNA homolog were positioned at the characteristic sites in plant PCNA genes. Attempts to amplify cDNA for an additional PCNA homolog from mammalian cells have been unsuccessful. These results suggest that the larger PCNA homolog was generated, presumably through gene duplication, after the divergence of the Planta and Animalia.

The gene for proliferating cell nuclear antigen (Pcna) maps to mouse Chromosome 2

The structural gene for proliferating cell nuclear antigen (Pcna) has been mapped to mouse Chromosome (Chr) 2 by use of a PCR-based assay. With somatic cell hybrids, Pcna was mapped between the T(2;4)13H and T(2;4)1Sn breakpoints. An interspecific backcross was employed to map Pcna 1.9 +/- 1.3 cM distal to Il-lb. This was confirmed by mapping Pcna in the BXH recombinant inbred (RI) strains; no recombinants were seen between Il-la and Pcna. In addition, a PCNA-related sequence (Pcna-rsl) was mapped to Chr 19 in the BXH RI strains.

Eukaryotic DNA replication

The past decade has witnessed an exciting evolution in our understanding of eukaryotic DNA replication at the molecular level. Progress has been particularly rapid within the last few years due to the convergence of research on a variety of cell types, from yeast to human, encompassing disciplines ranging from clinical immunology to the molecular biology of viruses. New eukaryotic DNA replicases and accessory proteins have been purified and characterized, and some have been cloned and sequenced. In vitro systems for the replication of viral DNA have been developed, allowing the identification and purification of several mammalian replication proteins. In this review we focus on DNA polymerases alpha and delta and the polymerase accessory proteins, their physical and functional properties, as well as their roles in eukaryotic DNA replication.

Primary structure of the catalytic subunit of human DNA polymerase delta and chromosomal location of the gene

The catalytic subunit (Mr approximately 124,000) of human DNA polymerase delta has been cloned by PCR using poly(A)+ RNA from HepG2 cells and primers designed from the amino acid sequence of regions highly conserved between bovine and yeast DNA polymerase delta. The human cDNA was 3443 nucleotides in length and coded for a polypeptide of 1107 amino acids. The enzyme was 94% identical to bovine DNA polymerase delta and contained the numerous highly conserved regions previously observed in the bovine and yeast enzymes. The human enzyme also contained two putative zinc-finger domains in the carboxyl end of the molecule, as well as a putative nuclear localization signal at the amino-terminal end. The gene coding for human DNA polymerase delta was localized to chromosome 19.

Proliferating cell nuclear antigen in developing and adult rat cardiac muscle cells

During early development, rat cardiac muscle cells actively proliferate. Shortly after birth, division of cardiac muscle cells ceases, whereas DNA synthesis continues for approximately 2 weeks at a progressively diminishing rate. Little DNA synthesis or cell division occurs in adult cardiocytes. Thus, developing cardiac muscle cells are an ideal system in which to examine the expression of cell cycle-regulated genes during development. We chose to examine proliferating cell nuclear antigen (PCNA), a gene expressed at the G1/S phase boundary of the cell cycle. Northern blots of RNA from cardiac muscle cells from 18-day-old rat fetuses and from day 0, 5, and 14 neonatal as well as adult rat hearts revealed that the PCNA mRNA was found in cardiac muscle cells from all ages. However, because it was possible that this was a result of fibroblast PCNA gene expression, we used reverse transcription followed by polymerase chain reaction to see if it was possible to detect the message for PCNA in cardiac muscle cells from all ages. Because of the great sensitivity of this technique, RNA was recovered from 25 isolated adult cardiac muscle cells. Polymerase chain reaction amplification products for PCNA produced from the RNA isolated from these cells conclusively demonstrated that mRNA for this gene, which normally is associated with proliferating cells, is expressed in adult cardiac muscle cells that no longer divide. Furthermore, Western blot analysis demonstrated that the PCNA protein was found only in embryonic and neonatal cells and not in adult rat cardiac muscle cells. Therefore, it might be inferred from these data that PCNA might be regulated at the posttranscriptional level in adult cardiac muscle cells.

Molecular cloning and structural analysis of mouse gene and pseudogenes for proliferating cell nuclear antigen

We have isolated clones containing the entire mouse proliferating cell nuclear antigen (PCNA) gene of 3890 bp and flanking sequences using a rat PCNA cDNA as a probe. The mouse gene has 6 exons whose sequences and junction points of exons with introns are extensively homologous to the human gene while sizes and nucleotide sequences of introns are much less conserved than exons. By a transient expression assay of chloramphenicol acetyltransferase, the promoter of this gene is localized within 200 bp upstream of the transcription initiation site. We have also isolated two processed pseudogenes. Homology between the first one (psi PCNA-I) and the exons of the PCNA gene was 76.8% in the region so far sequenced. The second one (psi PCNA-II) consists of a region highly homologous to the entire exons of the PCNA gene, and only 9 out of total 1256 bp are different from the corresponding exon sequence of the gene. The 5'-flanking region of the psi PCNA-II did not function as an active promoter. Surveys in various wild and laboratory mice genomes suggest that the psi PCNA-II was generated through the reverse transcription process of the PCNA mRNA about 5 x 10(5) years ago in the domesticus subspecies of Mus musculus, the house mouse. The psi PCNA-II is tentatively mapped in the chromosome 17 of the C57BL mouse.

Characterization of an enhancer-like structure in the promoter region of the proliferating cell nuclear antigen (PCNA) gene

The steady-state mRNA levels of the proliferating cell nuclear antigen (PCNA) gene depend on the length of its promoter. A promoter extending from the HpaII restriction site at -210 from the cap site to the cap site itself is very active, while a -45 promoter (AatII restriction site) is very weak. We now show that the sequences between -73 and -45 of the human PCNA promoter contain an enhancer-like sequence that markedly increases the levels of PCNA mRNA. This sequence has characteristics of an enhancer, having an enhancing function also when placed away from the native position in the 5' flanking sequence. The increase in mRNA levels that occurs after serum stimulation, however, is independent of the enhancer. Synthetic promoters were also constructed containing mutations in the -73 to -45 sequence and these mutants completely lost their ability to drive the transcription of a heterologous cDNA. Nuclear proteins were shown to bind to this sequence, both by gel shift and by methylation interference analysis. We conclude that the levels of PCNA mRNA are controlled, in part, by a structure located in the 5' flanking sequence of the gene, but that this enhancer-like structure does not play a role in the serum regulation of the mRNA levels.

Fanconi anemia: evidence for linkage heterogeneity on chromosome 20q

Fanconi anemia is a rare autosomal recessive disorder in which affected individuals are predisposed to acute myelogenous leukemia and other malignancies. We report the results of a genetic linkage study involving 34 families enrolled in the International Fanconi Anemia Registry. A significant lod score was obtained between D20S20, an anonymous DNA segment from chromosome 20q, and Fanconi anemia (Zmax 3.04, theta max = 0.12). However, six other anonymous DNA segments from chromosome 20q, including D20S19, which is highly polymorphic and tightly linked to D20S20, showed no or only weak evidence for linkage to Fanconi anemia. An admixture test revealed significant evidence for linkage heterogeneity (chi 2 = 6.10, P = 0.01) at the D20S19 locus. Lod scores suggestive of linkage between Fanconi anemia and this locus were obtained with two of the largest kindreds studied (lods = 2.6 and 2.1, at theta = 0.001). Thus, our data support the provisional assignment of a Fanconi anemia gene to chromosome 20q.

Localization of the gene for human proliferating nuclear antigen/cyclin by in situ hybridization

Proliferating cell nuclear antigen (PCNA)/cyclin has been localized by in situ hybridization to the short arm of human chromosome 20 with a peak of grains over band 20p13. In addition, there were two strong secondary peaks of grains over 11p15.1 and Xp11.4 indicating the presence of two related genes in man.

Cloning of three human tyrosine phosphatases reveals a multigene family of receptor-linked protein-tyrosine-phosphatases expressed in brain

A human brainstem cDNA library in bacteriophage lambda gt11 was screened under conditions of reduced hybridization stringency with a leukocyte common antigen (LCA) probe that spanned both conserved cytoplasmic domains. cDNA encoding a receptor-linked protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48), RPTPase alpha, has been cloned and sequenced. Human RPTPase alpha consists of 802 amino acids. The extracellular domain of 150 residues includes a hydrophobic signal peptide and eight potential N-glycosylation sites. This is followed by a transmembrane region and two tandemly repeated conserved domains characteristic of all RPTPases identified thus far. The gene for RPTPase alpha has been localized to human chromosome region 20pter-20q12 by analysis of its segregation pattern in rodent-human somatic cell hybrids. Northern blot analysis revealed the presence of two major transcripts of 4.3 and 6.3 kilobases. In addition to RPTPase alpha, two other RPTPases (beta and gamma), identified in the same screen, have been partially cloned and sequenced. Analysis of sequence comparisons among LCA, the LCA-related protein LAR, and RPTPases alpha, beta, and gamma reveals the existence of a multigene family encoding different RPTPases, each containing a distinct extracellular domain, a single hydrophobic transmembrane region, and two tandemly repeated conserved cytoplasmic domains.

The promoter of the human proliferating cell nuclear antigen (PCNA) gene is bidirectional

The proliferating cell nuclear antigen (PCNA) gene codes for a protein that is necessary for cellular DNA synthesis and cell cycle progression. A functional promoter has been identified in the 5' flanking region of the human PCNA gene. An abbreviated promoter (from the capsite to the PvuII restriction site at -395) was found to be equally efficient in directing transcription from a linked reporter, whether placed in the correct or reverse orientation in respect to the coding sequence. The reporter used was a cDNA of human thymidine kinase (TK), and the bidirectionality of the promoter was demonstrated by its ability to confer the TK+ phenotype to TK- ts 13 cells and by the amount of specific message in RNA blots. The PvuII promoter placed between two coding sequences (the TK cDNA and the bacterial gene for neoresistance) is capable of driving transcription simultaneously in both directions. Finally, in blots of RNA from human cells, two transcripts could be detected that hybridized to a sense riboprobe from the 5' flanking region of the human PCNA gene. We conclude that the locus for the human PCNA gene contains a bidirectional promoter producing diverging transcripts.

Importance of introns in the growth regulation of mRNA levels of the proliferating cell nuclear antigen gene

The steady-state mRNA levels of the proliferating cell nuclear antigen (PCNA) gene are growth regulated. We have begun to identify the elements in the human PCNA gene that participate in its growth regulation by transfecting appropriate constructs in BALB/c3T3 cells. The results can be summarized as follows. (i) The 400 base pairs of the 5'-flanking sequence of the human PCNA gene upstream of the preferred cap site are sufficient for directing expression of a heterologous cDNA (S. Travali, D.-H. Ku, M. G. Rizzo, L. Ottavio, R. Baserga, and B. Calabretta, J. Biol. Chem. 264:7466-7472, 1989). (ii) Intron 4 is necessary for the proper regulation of PCNA mRNA levels in G0 cells. Removal of intron 4 leads to abnormally high levels of PCNA mRNA in serum-deprived cells, although the shortened PCNA gene with its own promoter is still responsive to serum stimulation. (iii) The presence of introns also increases the steady-state levels of PCNA mRNA in proliferating cells. These results are especially interesting for two reasons: (i) because of the extensive sequence similarities among introns and between introns and exons of the human PCNA gene, and (ii) because, usually, the presence of introns leads to increased expression, whereas in this case, removal of intron 4 caused an increase in mRNA levels, and this occurred only in quiescent cells.

Importance of introns in the growth regulation of mRNA levels of the proliferating cell nuclear antigen gene

The steady-state mRNA levels of the proliferating cell nuclear antigen (PCNA) gene are growth regulated. We have begun to identify the elements in the human PCNA gene that participate in its growth regulation by transfecting appropriate constructs in BALB/c3T3 cells. The results can be summarized as follows. (i) The 400 base pairs of the 5'-flanking sequence of the human PCNA gene upstream of the preferred cap site are sufficient for directing expression of a heterologous cDNA (S. Travali, D.-H. Ku, M. G. Rizzo, L. Ottavio, R. Baserga, and B. Calabretta, J. Biol. Chem. 264:7466-7472, 1989). (ii) Intron 4 is necessary for the proper regulation of PCNA mRNA levels in G0 cells. Removal of intron 4 leads to abnormally high levels of PCNA mRNA in serum-deprived cells, although the shortened PCNA gene with its own promoter is still responsive to serum stimulation. (iii) The presence of introns also increases the steady-state levels of PCNA mRNA in proliferating cells. These results are especially interesting for two reasons: (i) because of the extensive sequence similarities among introns and between introns and exons of the human PCNA gene, and (ii) because, usually, the presence of introns leads to increased expression, whereas in this case, removal of intron 4 caused an increase in mRNA levels, and this occurred only in quiescent cells.