DNA replication progresses on the periphery of nuclear aggregates formed by the BCL6 transcription factor

The BCL6 proto-oncogene, frequently alterated in non-Hodgkin lymphoma, enco des a POZ/zinc finger protein that localizes into discrete nuclear subdomai ns. Upon prolonged BCL6 overexpression in cells bearing an inducible BCL6 a llele (UTA-L cells), these subdomains apparently coincide with sites of DNA synthesis. Here, we explore the relationship between BCL6 and replication by both electron and confocal laser scanning microscopy, First, by electron microscope analyses, we found that endogenous BCL6 is associated with repl ication foci. Moreover, we show that a relatively low expression level of B CL6 reached after a brief induction in UTA-L cells is sufficient to observe its targeting to mid, late, and at least certain early replication foci vi sualized by a pulse-labeling with bromodeoxyuridine (BrdU). In addition, wh en UTA-L cells are simultaneously induced for BCL6 expression and exposed t o BrdU for a few hours just after the release from a block in mitosis, a nu clear diffuse BCL6 staining indicates cells in G(1), while cells in S show a more punctate nuclear BCL6 distribution associated with replication foci. Finally, ultrastructural analyses in UTA-L cells exposed to BrdU for vario us times reveal that replication progresses just around, but not within, BC L6 subdomains. Thus, nascent DNA is localized near, but not colocalized wit h, BCL6 subdomains, suggesting that they play an architectural role influen cing positioning and/or assembly of replication foci. Together with its pre viously function as transcription repressor recruiting a histone deacetylas e complex, BCL6 may therefore contribute to link nuclear organization, repl ication, and chromatin-mediated regulation.