Binding of Ikaros to the lambda5 promoter silences transcription through a mechanism that does not require heterochromatin formation.
The Ikaros family of proteins are DNA binding factors required for correct development of B and T lymphocytes. Cytogenetic studies have shown that these proteins form complexes with pericentromeric heterochromatin in B cells, and the colocalization of transcriptionally silent genes with these complexes suggests that Ikaros could silence transcription by recruiting genes to heterochromatin. Here we show that a site in the lambda5 promoter that binds Ikaros and Aiolos is required for silencing of lambda5 expression in activated mature B cells. Analysis of methylation and nuclease accessibility indicates that the silenced lambda5 gene is not heterochromatinized in B cells, despite being associated with pericentromeric heterochromatin clusters. We also found that a promoter mutation, which affects Ikaros-mediated silencing of lambda5 expression, is not rescued in a transgenic line that has the gene integrated into pericentromeric heterochromatin. Our results indicate that the Ikaros proteins initiate silencing of lambda5 expression through a direct effect on the promoter with localization to pericentromeric heterochromatin likely to affect the action of Ikaros on regulatory sequences rather than causing heterochromatinization of the gene.
Sabbattini
P
P
Gene Regulation and Chromatin Group and Central Research Facility, MRC Clinical Sciences Centre, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London W12 ONN, UK. pierangela.sabbattini@csc.mrc.ac.uk
Lundgren
M
M
Georgiou
A
A
Chow
C
C
Warnes
G
G
Dillon
N
N
D000818:Animals; D001402:B-Lymphocytes / Q000502:physiology* / Q000648:ultrastructure; D001483:Base Sequence; D001665:Binding Sites; D002460:Cell Line; D004268:DNA-Binding Proteins; D020868:Gene Silencing; D006412:Hematopoietic Stem Cells / Q000502:physiology / Q000648:ultrastructure; D006570:Heterochromatin / Q000502:physiology*; D006801:Humans; D051740:Ikaros Transcription Factor; D007146:Immunoglobulin lambda-Chains / Q000235:genetics*; D008099:Liver / Q000196:embryology / Q000276:immunology; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013154:Spleen / Q000276:immunology; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*; D014158:Transcription, Genetic; D016335:Zinc Fingers
D004268:DNA-Binding Proteins; D006570:Heterochromatin; C497239:IKZF1 protein, human; D007146:Immunoglobulin lambda-Chains; D014157:Transcription Factors; C497238:Zfpn1a1 protein, mouse; D051740:Ikaros Transcription Factor