Binding of Ikaros to the lambda 5 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 the se proteins form complexes with pericentromeric heterochromatin in B cells, and the colocalization of transcriptionally silent genes with these comple xes suggests that Ikaros could silence transcription by recruiting genes to heterochromatin, Here we show that a site in the lambda5 promoter that bin ds Ikaros and Aiolos is required for silencing of lambda5 expression in act ivated mature B cells. Analysis of methylation and nuclease accessibility i ndicates that the silenced lambda5 gene is not heterochromatinized in B cel ls, despite being associated with pericentromeric heterochromatin clusters. We also found that a promoter mutation, which affects Ikaros-mediated sile ncing of lambda5 expression, is not rescued in a transgenic line that has t he gene integrated into pericentromeric heterochromatin, Our results indica te that the Ikaros proteins initiate silencing of lambda5 expression throug h a direct effect on the promoter with localization to pericentromeric hete rochromatin likely to affect the action of Ikaros on regulatory sequences r ather than causing hetero-chromatinization of the gene.