The Myc oncoprotein represses initiator-dependent transcription through the POZ domain transcription factor Miz-1. We now show that transactivation by Miz-1 is negatively regulated by association with topoisomerase II binding protein (TopBP1); UV irradiation downregulates expression of TopBP1 and releases Miz-1. Miz-1 binds to the p21Cip1 core promoter in vivo and is required for upregulation of p21Cip1 upon UV irradiation. Using both c-myc(-/-) cells and a point mutant of Myc that is deficient in Miz-1 dependent repression, we show that Myc negatively regulates transcription of p21Cip1 upon UV irradiation and facilitates recovery from UV-induced cell cycle arrest through binding to Miz-1. Our data implicate Miz-1 in a pathway that regulates cell proliferation in response to UV irradiation.
DNA polymerase epsilon (Pol epsilon) is believed to play an essential catalytic role during eukaryotic DNA replication and is thought to participate in recombination and DNA repair. That Pol epsilon is essential for progression through S phase and for viability in budding and fission yeasts is a central element of support for that view. We show that the amino-terminal portion of budding yeast Pol epsilon (Pol2) containing all known DNA polymerase and exonuclease motifs is dispensable for DNA replication, DNA repair, and viability. However, the carboxy-terminal portion of Pol2 is both necessary and sufficient for viability. Finally, the viability of cells lacking Pol2 catalytic function does not require intact DNA replication or damage checkpoints.
Topoisomerase II-binding protein (TopBP1), a human protein with eight BRCT domains, is similar to Saccharomyces cerevisiae Dpb11 and Schizosaccharomyces pombe Cut5 checkpoint proteins and closely related to Drosophila Mus101. We show that human TopBP1 is required for DNA replication and that it interacts with DNA polymerase ⑀. In S phase TopBP1 colocalizes with Brca1 to foci that do not represent sites of ongoing DNA replication. Inhibition of DNA synthesis leads to relocalization of TopBP1 together with Brca1 to replication forks, suggesting a role in rescue of stalled forks. DNA damage induces formation of distinct TopBP1 foci that colocalize with Brca1 in S phase, but not in G 1 phase. We also show that TopBP1 interacts with the checkpoint protein hRad9. Thus, these results implicate TopBP1 in replication and checkpoint functions. DNA polymerases (pol)1 play essential roles in chromosomal DNA replication and repair. In Saccharomyces cerevisiae three essential nuclear polymerases, ␣, ␦, and ⑀ have important functions in DNA replication. S. cerevisiae pol ⑀ is isolated as a complex of a catalytic subunit and three smaller subunits, Dpb2, 3, and 4 (1). This four-subunit structure is also conserved in the human enzyme, which consists of a catalytic subunit (2), a B subunit (3, 4), and two smaller subunits (5). Pol ⑀ is a proofreading DNA polymerase, which has been implicated in DNA replication, as temperature-sensitive mutants show defects in DNA replication in both S. cerevisiae and Schizosaccharomyces pombe (6 -8). Moreover, pol ⑀ is associated with origins of DNA replication and it proceeds along the replication fork (9). In human cells, pol ⑀ is associated with actively replicated cellular DNA (10) and has been shown to perform an important fraction of replicative DNA synthesis (11). Surprisingly, the catalytic domain of pol ⑀ is not essential for viability in S. cerevisiae. Instead, the C terminus, which interacts with Dpb2, exerts all of the essential functions (12).Pol ⑀ has been proposed to function in the repair of UVdamaged DNA because it is able to catalyze UV-induced DNA synthesis in vivo (13) and performs efficient gap-filling synthesis in the reconstituted nucleotide excision repair system (14). A role in base excision repair is suggested by the fact that pol ⑀ mutants fail to support repair synthesis in vitro, and repair activity can be restored by the addition of purified pol ⑀ (15). Pol ⑀ has also been proposed to function in a specialized replication process required to repair double strand breaks (16). In addition to replicative and repair roles, it has been suggested that pol ⑀ coordinates transcriptional and cell cycle responses to DNA damage and replication blocks (17).In S. cerevisiae, a BRCT domain-containing protein, Dpb11, interacts with the pol ⑀ complex and was originally identified as a suppressor of pol ⑀ catalytic and Dpb2 subunit mutants (18,19). DPB11 is an essential gene required for DNA replication (18). The inability of DPB11 mutants to restrain mitosis in the presence of inco...
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