Additional support for a role of HMGN in
Additional support for a role of HMGN1 in enhancing DNA repair comes from studies of transcription-coupled repair (TCR) in human cells. Lesions in the template strand of DNA induced by UV light stall the movement of RNA polymerase II (RNAPII) and interrupt transcription. Removal of these transcription-blocking lesions by TCR is required for the resumption transcription elongation. In mammals, TCR is dependent on Cockayne syndromes A and B (CSA and CSB) proteins. Recent investigations by Fousteri and his colleagues  have demonstrated that CSA and CSB play different roles in recruiting TRC-specific factors to UV-stalled RNAPII during repair complex formation. The CSB protein plays an essential role in lesion repair by acting as a coupling factor that attracts other repair factors (including HAT p300, NER proteins and a ubiquitin ligase/signalosome complex of proteins containing CSA) to the stalled RNAPII. Interestingly, CSA is not necessary for attraction of NER proteins to lesion-stalled RNAPII and yet, in cooperation with CSB, is required to recruit HMGN1 and certain transcriptions factors to the repair complex. Thus, recruitment of HMGN1 to stalled RNAPII complex is a relatively late event during TCR and likely plays a role in altering chromatin structure to facilitate lesion removal and the re-initiation of transcription .
Conclusions: three's company Fluorescence recovery after photobleaching (FRAP) techniques have demonstrated that in living ha tag peptide the interaction of chromatin binding proteins with their nuclear targets is dynamic and transient, with many of the proteins diffusing through the entire nucleus on a time scale of seconds . Photobleaching experiments have also shown that there are highly dynamic, three-dimensional networks of protein–chromatin interactions in the nucleus that influence chromatin structure on a global scale . Employing cell microinjection and FRAP techniques, Catez et al.  investigated whether members of the different HMG families could compete with histone H1 for binding to chromatin substrates inside cells, as would be predicted from the results of numerous in vitro experiments. Purified recombinant proteins from each of the three HMG families (HMGA, HMGB and HMGN) were microinjected into the cytoplasm of cells expressing transgenic histone H1-green fluorescent protein (H1-GFP) and FRAP was used to compare the mobility of H1-GFP in injected cells to that in uninjected control cells. The results showed that proteins from each of the three HMG families weakened the binding of H1 to nucleosomes by dynamically competing for chromatin binding sites in vivo. Furthermore, it was found that the different HMG families do not compete with each other for chromatin binding. Rather, the HMG families act synergistically to weaken H1 binding to chromatin, results that suggest each HMG type competes with H1 for unique binding sites and for the formation of distinct protein complexes around different nucleosome linker DNAs. Based on these findings, Catez et al. proposed that a network of dynamic and competitive interactions involving HMG proteins and H1, and perhaps other structural proteins, constantly modulates nucleosome accessibility and the local structure of the chromatin fiber. As a consequence of such competition, H1 displacement by HMGs loosens up the higher-order structure of the chromatin fiber thereby allowing a “window of opportunity” for regulatory factors to access previously restricted regions and to modulate gene transcription .
Note added in proof Recently Rochman et al. demonstrated that the negatively charged C-terminal domain of HMGN5 (a.k.a., NSBP1/NBP-45) targets this newly discovered member of the HMGN family to nucleosomes in the euchromatic regions of nuclei and alters the compaction of chromatin in living mouse cells. These workers further established that, in vitro, HMGN5 interacts with the positively charged C-terminal tail of histone H5 and counteracts the compaction of a nucleosomal array by this linker histone. Additionally it was shown that induced alterations in the cellular levels of HMGN5 modulate the transcription level of numerous genes in vivo. Together these findings support a model in which HMGN5 modulates the transcriptional profile of cells by localizing to euchromatic regions and counteracting the chromatin-condensing activity of linker histones.