Thus, histone displacement in this instance occurred by transfer of complete histone octamers, a mechanism distinct from that mediated by the histone-binding proteins nucleoplasmin and NAP-1. By contrast, transcription factor-induced histone transfer onto nonspecific competitor DNA did not require disruption of the histone octamer. These data indicate that octamer disruption is required for both stimulation of factor binding and factor-dependent histone displacement by nucleoplasmin and NAP-1. In addition, both NAP-1 and nucleoplasmin were able to mediate histone displacement upon the binding of five GAL4-AH dimers to control nucleosome cores however, this activity was also forfeited when the histone octamers were cross-linked. While NAP-1 and nucleoplasmin stimulated the binding of a fusion protein (GAL4-AH) to control nucleosome cores, this stimulation was lost upon covalent histone-histone cross-linking within the histone octamers. We show that none of these is able to locate chicken muscle pyruvate kinase to the cell nucleus. In addition, disruption of the histone octamer was required both for the stimulation of transcription factor binding to nucleosomal DNA and for transcription factor-induced nucleosome displacement mediated by nucleoplasmin or NAP-1. The carboxy-terminal tail of nucleoplasmin, which specifies entry into the cell nucleus, contains four short sequences that are similar to previously identified nuclear location sequences. We found that yeast nucleosome assembly protein 1 (NAP-1) stimulated transcription factor binding and nucleosome displacement in a manner similar to that of nucleoplasmin.

To investigate the mechanisms by which transcription factors invade nucleosomal DNA and replace histones at control elements, we have examined the response of the histone octamer to transcription factor binding in the presence of histone-binding proteins (i.e., nucleosome assembly factors).

Nucleoplasmin