The BCL6 promoter is a target for BCL6-mediated transcriptional repression.

(A) Negative regulation of endogenous BCL6 by constitutive expression of exogenous BCL6 in Ramos cells. Northern blot analysis of endogenous (endo) and exogenous (exo) BCL6 expression in Ramos cells and in Ramos clones transduced with the PINCO-HA-BCL6 retroviral vector. The signal intensity ratio between endogenous BCL6 and GAPDH, quantitated by PhosphorImager analysis, is shown in the lower panel. (B) CdCl2-induced down-regulation of endogenous BCL6 gene expression in EREB cells stably transfected with an inducible BCL6 gene. Northern blot analysis of EREB cells transfected with MT (EREB-MT) or MT-BCL6 (EREB-MT-BCL6) plasmids. Cells were treated with CdCl2 as described in “Materials and methods” and were collected at the indicated intervals. The Ramos cell line was included as a control for size of the endogenous BCL6 transcripts. Filters were sequentially hybridized with a radiolabeled BCL6 cDNA probe and with GAPDH to control for amount of RNA loading. (C) Schematic representation of the plasmids used in transient cotransfection experiments. POZ indicates protein–protein interaction domain; and ZF, zinc finger DNA-binding domain. (D) The reporter construct indicated in panel C was transfected alone or in the presence of increasing amounts of various BCL6-expressing plasmids into 293T cells. Luciferase activities measured 48 hours after transfection revealed a strong and dose-dependent repression of the reporter gene when cotransfected with the wild-type BCL6-expressing plasmid (solid bars), but not with 2 deletion mutants that lack the DNA-binding (ΔZF) or the transrepression domain (ZF) (hatched bars). All experiments were performed in duplicate, and standard deviations are indicated.