Differentiation-dependent Chromatin Alterations Precede and Accompany Transcription of Immunoglobulin Light Chain Genes

We have studied the nature of chromatin alterations along immunoglobulin light chain (IgL) genes during B cell development using cultured murine cell lines. Employing a chromatin fractionation procedure on micrococcal nuclease-treated nuclei, we demonstrate that transcriptionally active k IgL chromatin lacks a canonical nucleosomal repeat and exhibits a pronounced association with insoluble nuclear material but is processed by nuclease to a soluble nucleosomal component that apparently lacks histone H1 and is enriched in high mobility group proteins. Of particular significance, utilizing a variant plasmacytoma cell line that has transcriptionally inactivated one k allele via a promoter deletion, we demonstrate that transcription per se is not responsible for these novel alterations. Furthermore, we show that the chromatin encompassing germline (unrearranged) and transcriptionally silent lambda IgL alleles in k-producing plasmacytomas exhibit some of the same unusual properties that are displayed by k alleles. Finally, we demonstrate that these alterations only occur in cell lines of the lymphocyte lineage that have progressed past the early pre-B cell stage; when inactive, both k and lambda IgL genes possess typical nucleosomal packaging and co-fractionate with histone H1-containing chromatin. These findings lead us to propose a model that predicts B cell stage-specific alterations in IgL chromatin prior to gene rearrangement and transcription.