Chromatin structure: The role of “linker” proteins

Abstract

Structural organization of chromatin in eukaryotic cells plays an important role in the functioning of the genetic ap- paratus of the cell. The nucleoprotein complex is formed due to the interactions of DNA with histones and non-histone proteins. Here we describe the main levels of the structural organization of the chromatin. Special attention is paid to the spectroscopic techniques such as infrared absorption spectroscopy, electronic and vibrational circular dichroism and their impact to the in- vestigation of the chromatin structure. The linker proteins: histone H1 and non-histone protein HMGB1 and their role in the formation of the super compact chromatin is discussed. The chromatin of the eukaryotic cells comprises of DNA and variety of nuclear proteins. The linear dimensions of eukaryotic DNA molecule is much larger than the diameter of the cell (11). That is why one of the important functions of the chromatin is DNA compaction. The subsequent levels of DNA compaction are associated with the changes of chromatin structure resulting in a high density of DNA packing in the cell nucleus. The structural organization of the DNA in complexes with the nuclear pro- teins plays an important role, as it determines not only the structure of chromatin, but also the functioning of the genetic apparatus of the cell (3,32). The nucleoprotein complex is formed due to the interactions of DNA with histones and non-histone proteins (11). The proteins functioning in the linker regions of the chromatin (linker proteins) are the key elements of the DNA compaction (26). The most abundant linker proteins are histone H1 and HMGB non-histone proteins. The DNA-binding properties of these proteins determine the differences in the structural organization of the chromatin in the cells of different organisms, modulating the function- ing of their genomes (11,12,15,53). The HMGB1 protein is considered to be an architectural factor of transcription (9,35,63,80), and participates in transcription (1,8,23,63), recombination (66,79) and replication (67).