CLUSTERS OF NUCLEOSOMES CONTAINING CHROMOSOMAL PROTEIN HMG-17 IN CHROMATIN

Chromosomal proteins HMG-14 and HMG-17 are nucleosome binding proteins which can function as architectural elements to alter the structure o f the chromatin fiber and enhance transcription from chromatin templat es. Here we study the spatial organization of these HMG proteins in th e nucleus and the distribution of nucleosomes containing HMG-17 in the chromatin fiber. By confocal immunofluorescence microscopy we find tl -rat HMG-14/17 proteins are clustered into foci containing either HMG- 14 or HMG-17. These results suggest that HMG-14/17 proteins segregate into distinct nuclear domains. Indeed, immunofractionation of defined length oligonucleosomes, with affinity pure antibodies to HMG-17, indi cates that oligonucleosomes containing HMG-17 are devoid of HMG-14. Qu antitative analysis indicates that in cellular chromatin nucleosomes c ontaining HMG-17 are clustered. The average size of the cluster is six contiguous HMG-17-containing nucleosomes. The nucleosomes in this clu ster contain either two or zero molecules of HMG-17 and a complete set of four core histones. We suggest that HMG-14/17 proteins modify the nucleosomal organization of the 30 nm chromatin fiber, to unfold the h igher order chromatin structure and facilitate access to the underlyin g DNA sequence. Clustering of architectural elements, such as HMG prot eins and linker histone subtypes into distinct domains, may lead to st ructural and functional heterogeneity along the chromatin fiber. (C) 1 997 Academic Press Limited.