ALTERATIONS IN NUCLEOSOME CORE STRUCTURE IN LINKER HISTONE-DEPLETED CHROMATIN

We have previously shown that the sequential arrangement of histone-DN A contacts is essentially the same in the nucleosomal core of sea urch in sperm nuclei, where chromatin is highly condensed and repressed, an d in nuclei hom lily bud sepals or yeast, where chromatin is highly ac tive in transcription and replication and is significantly or complete ly depleted of histone H1, However, the difference in the strength of some histone-DNA contacts has not been understood or discussed, In thi s work, we demonstrate that some of these differences are due to a con formational change in the nucleosomal core. We show that the nucleosom al core in linker histone-depleted chromatin is in a different conform ational state compared with the nucleosomal core in folded chromatin o r in isolated core nucleosomes, This conformational state is character ized by altered strengths in the histone H4 and H2A/H2B contacts with the regions of sharply bent nucleosomal DNA around sites +/-1 and +/-4 and site +/-5, respectively, We demonstrate that this conformation, w hich we call the ''stretched nucleosome,'' is a general feature of unf olded linker histone-depleted chromatin and may occur during chromatin activation, Our results suggest that this nucleosome structural alter ation does not depend on chromatin sources and histone variants studie d in this work, In addition, we show that this alteration is reversibl e and is caused by the stretching of Linker DNA during chromatin unfol ding.