Binding of AlF-C, an Orc1-binding transcriptional regulator, enhances replicator activity of the rat aldolase B origin

A region encompassing the rat aldolase B gene (aldB) promoter acts as a chromosomal origin of DNA replication (origin) in rat aldolase B-nonexpressing hepatoma cells. To examine replicator function of the aldB origin, we constructed recombinant mouse cell lines in which the rat aldB origin and the mutant derivatives were inserted into the same position at the mouse chromosome 8 by cre-mediated recombination. Nascent strand abundance assays revealed that the rat origin acts as a replicator at the ectopic mouse locus. Mutation of site C in the rat origin, which binds an Orc1-binding protein AlF-C in vitro, resulted in a significant reduction of the replicator activity in the mouse cells. Chromatin immunoprecipitation (ChIP) assays indicated that the reduction of replicator activity was paralleled with the reduced binding of AlF-C and Orc1, suggesting that sequence-specific binding of AlF-C to the ectopic rat origin leads to enhanced replicator activity in cooperation with Orc1. Involvement of AlF-C in replication in vivo was further examined for the aldB origin at its original rat locus and for a different rat origin identified in the present study, which contained an AlF-C-binding site. ChIP assays revealed that both replication origins bind AlF-C and Orc1. We think that the results presented here may represent one mode of origin recognition in mammalian cells.     

A binding site for Pur alpha and Pur beta is structurally unstable and is required for replication in vivo from the rat aldolase B origin

The rat aldolase B promoter acts as a replication origin in vivo, as well as an autonomously replicating sequence (ARS). Here, we examined roles of a polypurine stretch (site PPu) in this origin, which is indispensable to the ARS activity. Purification of site PPu-binding protein revealed that site PPu binds Puralpha and Purbeta, i.e., single-stranded DNA-binding proteins whose roles in replication have been implicated, but less clear. Biochemical analyses showed that site PPu even in a longer DNA fragment is unstable in terms of double-helix, implying that Puralpha/beta may stabilize single-stranded state. Deletion of site PPu from the origin DNA, which was ectopically positioned in the mouse chromosome, significantly reduced replicator activity. Chromatin immunoprecipitation experiments showed that deletion of site PPu abolishes binding of the Puralpha/beta proteins to the origin. These observations suggest functional roles of site PPu and Puralpha/beta proteins in replication initiation.