Abstract
The high mobility group N (HMGN) proteins, which were discovered over 50 years ago, are a multigene family of abundant nucleosome-specific binding factors that are present in all vertebrates. Despite their intriguing nucleosome-binding activity, the potential functions of the HMGN proteins in chromatin have not yet been assessed unambiguously due to the presence of several related HMGN genes in vertebrates and the lack of HMGN null cells. Here, we investigated the genome-wide activities of the human HMGN proteins by generating and analyzing an HMGN null cell line and isogenic HMGN rescue cell lines. These experiments revealed that the HMGN proteins function in the activation of gene expression at the level of transcription initiation at over a thousand specific sites that are mostly in promoters and enhancers. We additionally observed shared as well as unique functions of HMGN1 and HMGN2, which are likely to be the most abundant and ancient HMGN proteins. These findings thus indicate that the HMGN nucleosome-binding proteins are vertebrate-specific regulatory factors that primarily function in the activation of transcription initiation. Hence, any comprehensive model of vertebrate gene regulation should incorporate the contributions of the HMGN proteins, which are integral components of chromatin in all vertebrates.