Chromatin-associated HMGA and HMGB proteins: versatile co-regulators of DNA-dependent processes

High-mobility-group (HMG) proteins are small and relatively abundant chromatin-associated proteins, which act as architectural factors. In plants, two groups of chromosomal HMG proteins have been identified, namely the HMGA family, typically containing four A/T-hook DNA-binding motifs, and the HMGB family, containing a single HMG-box DNA-binding domain. The HMGA proteins are structurally flexible and bind A/T-rich DNA stretches. By orchestrating multiple protein-protein and protein-DNA interactions, they assist the formation of higher-order transcription factor complexes, regulating gene expression. The HMGB proteins bind DNA non-sequence-specifically, but specifically recognise DNA structures. Due to their remarkable DNA bending activity, they can enhance the structural flexibility of DNA, facilitating the assembly of nucleoprotein structures that control various DNA-dependent processes such as transcription and recombination.