Stimulation of human neuroblastoma DNA polymerase alpha and primase activities by a protein factor isolated from rat liver chromatin

Replicative enzymes and ageing: importance of DNA polymerase alpha function to the events of cellular ageing

A hallmark of cellular ageing is the failure of senescing cells to initiate DNA synthesis and transition from G1 into S phase of the cell cycle. This transition is normally dependent on or concomitant with expression of a set of genes specifying cellular proteins, some of which directly participate in DNA replication. Deregulation of this gene expression may play a pivotal role in the ageing process. The number of known enzymes and co-factors required to maintain integrity of the genome during eukaryotic DNA replication has increased significantly in the past few years, and includes proteins essential for DNA replication and repair, as well as for cell cycle regulation. In eukaryotic cells, ranging from yeast to man, a replicative enzyme essential for initiation of transcription is DNA polymerase alpha (pol alpha), the activity of which is coordinately regulated with the initiation of DNA synthesis. DNA pol alpha, by means of its primase subunit, has the unique ability to initiate de novo DNA synthesis, and as a consequence, is required for the initiation of continuous (leading-strand) DNA synthesis at an origin of replication, as well as for initiation of discontinuous (lagging-strand) DNA synthesis. The dual role of the pol alpha-primase complex makes it a potential interactant with the regulatory mechanisms controlling entry into S phase. The purpose of this review is to address the regulation and/or modulation of DNA pol alpha during ageing that may play a key role in the cascade of events which ultimately leads to the failure of old cells to enter or complete S phase of the cell cycle.

Purification and characterization of a yeast DNA polymerase alpha complex with associated primase, 5'-->3' exonuclease, and DNA-dependent ATPase activities

We have purified a multimeric form of yeast DNA polymerase alpha with DNA polymerase, primase, 5'-->3' exonuclease, and single-stranded (ss) DNA-dependent ATPase activities to near-homogeneity. The molecular mass of complex was 650 kDa with subunits ranging in sizes from 30 to 180 kDa. The alpha-subunit of the complex could be detected by DNA polymerase alpha antibody. No cross-reactivity of polypeptides within the complex was observed with antibodies directed against polymerase delta or epsilon. The multimeric polymerase alpha could be selectively inhibited by p-n-butylphenyl-dGTP (I50 of approximately 0.2 microM), p-n-butylanilino-dATP (I50 of 1.3 microM), and aphidicolin (I50 of 2.5 micrograms/mL). The complex synthesized RNA primers on various ssDNA templates and rapidly elongated these primers into nascent DNA fragments in the presence of required deoxynucleotides. It has a strong 5'-->3' exonuclease activity. In addition, the complex hydrolyzed both ATP and dATP in a ssDNA-dependent manner. Thus, the multiprotein complex of DNA polymerase alpha had multiple activities (primase, polymerase, and ATPase) which could act concertedly to synthesize primers and elongate the primers to nascent DNA fragments in the lagging strand of the fork.

Molecular cloning of a gene encoding an ARS binding factor from the yeast Saccharomyces cerevisiae

We report the isolation of the gene for origin binding factor 1 (OBF1) from the yeast Saccharomyces cerevisiae by screening a yeast genomic DNA library in lambda gt11 with an ARS-specific oligonucleotide probe. One recombinant encoded a fusion protein of approximately 180 kDa that bound ARS-specific oligonucleotide probes in vitro. The restriction map of this gene was determined after isolation of the complete gene by screening a yeast genomic DNA library in YEp24. Characterization of the gene for OBF1 by pulsed-field gel electrophoresis and Northern and Southern blot analyses demonstrated that (i) the gene is located in chromosome IV, (ii) the gene is a single-copy gene, (iii) the mRNA is approximately 3.8 kilobases, which could code for an approximately 130-kDa polypeptide, consistent with the reported size of OBF1. An antibody, affinity-purified using the lysogen-encoded fusion protein, specifically detected an approximately 130-kDa polypeptide in yeast extract. The isolation of the gene for OBF1 should allow further analysis of the mechanism of action of this protein in vitro and in vivo.

Replicating premeiotic germ cells of the mouse contain a novel DNA primase stimulatory factor

A protein factor that stimulates DNA primase activity associated with DNA polymerase alpha has been identified in mouse germ cell populations enriched in spermatogonia and preleptotene spermatocytes. The partially purified factor enhances DNA primase activity from homologous cell types as well as DNA primase activity from Xenopus laevis oocytes in a poly dT or M 13 directed reaction. The factor does not stimulate DNA polymerase alpha activity in a gapped salmon sperm or poly dT-rA directed reaction. The DNA primase stimulating factor is identified in a male mouse germ cell population enriched in premeiotic cells; it is not detectable in middle-late pachytene spermatocytes, spermatids, Sertoli cells or fibroblasts.