NUCLEOSOME STRUCTURAL TRANSITION DURING CHROMATIN UNFOLDING IS CAUSEDBY CONFORMATIONAL-CHANGES IN NUCLEOSOMAL DNA

We have recently reported that certain core histone-DNA contacts are a ltered in nucleosomes during chromatin unfolding (Usachenko, S. I., Ga vin I. M., and Bavykin, S. G. (1996) J. Biol. Chem. 271, 3831-3836), I n this work, we demonstrate that these alterations are caused by a con formational change in the nucleosomal DNA, Using zero-length protein-D NA cross linking, we have mapped histone-DNA contacts in isolated core particles at ionic conditions affecting DNA stiffness, which may chan ge the nucleosomal DNA conformation, We found that the alterations in histone-DNA contacts induced by an increase in DNA stiffness in isolat ed core particles are identical to those observed in nucleosomes durin g chromatin unfolding. The change in the pattern of micrococcal nuclea se digestion of linker histone-depleted chromatin at ionic conditions affecting chromatin compaction also suggests that the stretching of th e linker DNA may alter the nucleosomal DNA conformation, resulting in a structural transition in the nucleosome which may play a role in ren dering the nucleosome competent for transcription and/or replication.