Nonhistones

Nonhistones are a group of proteins associated with chromatin that are not histones. Historically defined as the proteins in chromatin preparations that migrate differently on electrophoresis than histones, and are not histone proteins themselves. They include a broad set such as high mobility group (HMG) proteins and various nonhistone chromosomal proteins, transcription factors, chromatin remodeling factors, and enzymes like DNA polymerases and topoisomerases that associate with chromatin.

Functions of nonhistones include regulation of chromatin structure and gene expression. HMG proteins can bind DNA in a sequence-independent manner and induce bending or looping, facilitating the assembly of transcriptional complexes. Chromatin remodelers reposition or eject nucleosomes, enabling access to DNA by transcription machinery. Nonhistone proteins also participate in DNA replication, repair, recombination, and chromatin compaction.

Characteristics of this group are diverse. They are embedded in chromatin, often post-translationally modified, and their chromatin association is dynamic. Unlike histones, they do not form the conserved octamer-DNA structure, and their functions span a broad range of chromatin-related processes.

Modern usage and context emphasize that the term "nonhistones" is historical. Contemporary studies classify these proteins by families (for example, HMG proteins, transcription factors, chromatin remodelers) and focus on their specific interactions with chromatin. Abnormal expression or modification of some nonhistone chromosomal proteins can influence development and disease.