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...

DNA damage tolerance facilitates the progression of replication forks that have encountered obstacles on the template strands. It involves either translesion DNA synthesis initiated by proliferating cell nuclear antigen monoubiquitination or less well-characterized fork reversal and template switch mechanisms. Herein, we characterize a novel tolerance pathway requiring the tumor suppressor p53, the translesion polymerase ι (POLι), the ubiquitin ligase Rad5-related helicase-like transcription factor (HLTF), and the SWI/SNF catalytic subunit (SNF2) translocase zinc finger ran-binding domain containing 3 (ZRANB3). This novel p53 activity is lost in the exonucleasedeficient but transcriptionally active p53(H115N) mutant. Wild-type p53, but not p53(H115N), associates with POLι in vivo. Strikingly, the concerted action of p53 and POLι decelerates nascent DNA elongation and promotes HLTF/ZRANB3-dependent recombination during unperturbed DNA replication. Particularly after cross-linkerinduced replication stress, p53 and POLι also act together to promote meiotic recombination enzyme 11 (MRE11)-dependent accumulation of (phospho-)replication protein A (RPA)-coated ssDNA. These results implicate a direct role of p53 in the processing of replication forks encountering obstacles on the template strand. Our findings define an unprecedented function of p53 and POLι in the DNA damage response to endogenous or exogenous replication stress.T he tumor suppressor protein p53 has been called the guardianof-the-genome due to its ability to transactivate downstream targets transcriptionally, which prevents S-phase entrance before facilitating DNA repair or eliminating cells with severe DNA damage via apoptosis (1). Interestingly, p53 also encodes an intrinsic 3′-5′ exonuclease activity located within its central DNA-binding domain (2-4). The contribution of the exonuclease proficiency to p53's function has largely remained obscure. Exonucleases are involved in DNA replication, DNA repair, and recombination, increasing the fidelity or efficiency of these processes. The 3′-5′ exonuclease activity of DNA polymerases (POLs) catalyzes the correction of replication errors, thereby preventing genomic instability and cancer (5-7). The potential involvement of p53's exonuclease in DNA repair has been ascribed to transcription-independent functions in nucleotide excision repair and base excision repair, in homologous recombination (HR), and in mitochondrial processes (8-10).Regarding HR, in particular, reports indicate a dual role for p53. On the one hand, it has been reported that p53 down-regulates unscheduled and excessive HR in response to severe genotoxic stress, like formation of DNA double-strand breaks (DSBs) (8-10). This antirecombinogenic effect of p53 has been linked to the blockage of continued strand exchange by interactions with recombinase RAD51, RAD54, and nascent HR intermediates carrying specific mismatches (11, 12). On the other hand, p53 stimulates spontaneous HR during S-phase to overcome replication fork stalling and to pr...

Bacterial isolates were obtained from pine (Pinus sylvestris L.) tissue cultures and identified as Methylobacterium extorquens and Pseudomonas synxantha. The existence of bacteria in pine buds was investigated by 16S rRNA in situ hybridization. Bacteria inhabited the buds of every tree examined, primarily colonizing the cells of scale primordia and resin ducts.

Besides mutations in BRCA1/BRCA2, heterozygous defects in PALB2 are important in breast cancer predisposition. PALB2 heterozygosity increases the risk of malignancy about sixfold. PALB2 interacts with BRCA1 and BRCA2 to regulate homologous recombination and mediate DNA damage response. Here we show, by analysing lymphoblastoid cell lines from heterozygous female PALB2 mutation carriers, that PALB2 haploinsufficiency causes aberrant DNA replication/damage response. Mutation carrier cells show increased origin firing and shorter distance between consecutive replication forks. Carrier cell lines also show elevated ATR protein, but not phosphorylation levels, and a majority of them display aberrant Chk1-/Chk2-mediated DNA damage response. Elevated chromosome instability is observed in primary blood lymphocytes of PALB2 mutation carriers, indicating that the described mechanisms of genome destabilization operate also at the organism level. These findings provide a new mechanism for early stages of breast cancer development that may also apply to other heterozygous homologous recombination signalling pathway gene mutations in hereditary cancer predisposition.