Segregation of replicative DNA polymerases during S phase: DNA polymerase ε, but not DNA polymerases α/δ, are associated with lamins throughout S phase in human cells.

DNA polymerases (Pol) α, δ, and ε replicate the bulk of chromosomal DNA in eukaryotic cells, Pol ε being the main leading strand and Pol δ the lagging strand DNA polymerase. By applying chromatin immunoprecipitation (ChIP) and quantitative PCR we found that at G(1)/S arrest, all three DNA polymerases were enriched with DNA containing the early firing lamin B2 origin of replication and, 2 h after release from the block, with DNA containing the origin at the upstream promoter region of the MCM4 gene. Pol α, δ, and ε were released from these origins upon firing. All three DNA polymerases, Mcm3 and Cdc45, but not Orc2, still formed complexes in late S phase. Reciprocal ChIP of the three DNA polymerases revealed that at G(1)/S arrest and early in S phase, Pol α, δ, and ε were associated with the same nucleoprotein complexes, whereas in late S phase Pol ε and Pol α/δ were largely associated with distinct complexes. At G(1)/S arrest, the replicative DNA polymerases were associated with lamins, but in late S phase only Pol ε, not Pol α/δ, remained associated with lamins. Consistently, Pol ε, but not Pol δ, was found in nuclear matrix fraction throughout the cell cycle. Therefore, Pol ε and Pol α/δ seem to pursue their functions at least in part independently in late S phase, either by physical uncoupling of lagging strand maturation from the fork progression, or by recruitment of Pol δ, but not Pol ε, to post-replicative processes such as translesion synthesis or post-replicative repair.

DNA polymerase epsilon associates with the elongating form of RNA polymerase II and nascent transcripts.

DNA polymerase epsilon co-operates with polymerases alpha and delta in the replicative DNA synthesis of eukaryotic cells. We describe here a specific physical interaction between DNA polymerase epsilon and RNA polymerase II, evidenced by reciprocal immunoprecipitation experiments. The interacting RNA polymerase II was the hyperphosphorylated IIO form implicated in transcriptional elongation, as inferred from (a) its reduced electrophoretic mobility that was lost upon phosphatase treatment, (b) correlation of the interaction with phosphorylation of Ser5 of the C-terminal domain heptapeptide repeat, and (c) the ability of C-terminal domain kinase inhibitors to abolish it. Polymerase epsilon was also shown to UV crosslink specifically alpha-amanitin-sensitive transcripts, unlike DNA polymerase alpha that crosslinked only to RNA-primed nascent DNA. Immunofluorescence microscopy revealed partial colocalization of RNA polymerase IIO and DNA polymerase epsilon, and immunoelectron microscopy revealed RNA polymerase IIO and DNA polymerase epsilon in defined nuclear clusters at various cell cycle stages. The RNA polymerase IIO-DNA polymerase epsilon complex did not relocalize to specific sites of DNA damage after focal UV damage. Their interaction was also independent of active DNA synthesis or defined cell cycle stage.

Negative regulation of the mammalian UV response by Myc through association with Miz-1.

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.