Nucleosome chain compaction

The principles whereby a chain of nueleosomes can compact to form a 30 nm chromatin fiber are still not well understood. Nevertheless, important aspects of this process are becoming clear from imaging studies, employing both ECM and SFM. When isolated chicken erythrocyte chromatin or chromatin reconstituted onto six tandem 208 bp nucleosome positioning units were examined by ECM, a linker DNA stem-like architectural motif was observed at the entry-exit sites (Fig. 4) [30]. Particles consistent with an octamer are surrounded with 1.7 turns of DNA, a linker... 

The ability of the histone-like TAFs to form an octamer-like structure raises the possibility that the TAF octamer may wrap promoter DNA in a manner similar to the nucleosome (Hoffmann et al., 1997 Oelgeschlager et al., 1996). This hypothesis is supported by the resemblance of DNase I footprinting patterns of TFIID on the Adenovirus Major Late (AdML) promoter to those of nucleosomal DNA. However, the arginine side chains in histones that form primary contacts with DNA are not conserved in TAFs (Luger et al., 1997). Therefore, the histone fold domain interaction may be used only for the formation of a compact structure and is not necessarily involved in DNA wrapping. 

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