ИСТИНА

Using the 5C technique in chicken erythroid and lymphoid cells, we analyzed the spatial structure of a 2.7 Mb region of chicken chromosome 14 harboring the alpha-globin gene domain. Our results are the first to provide evidence for topologically associating domains (TADs) and A/B-like chromatin compartments in birds. We find that complete transcriptional activation of the alpha-globin genes in erythroblastoid HD3 cells induced to terminal differentiation correlates with a dramatic upregulation of neighboring non-globin genes as revealed by total RNA-seq, and with significant decompaction of the entire 300 Kb TAD T3 containing the alpha-globin genes. In B-lymphoblastoid DT40 cells, this TAD is divided into two smaller domains that are separated by a CTCF-occupied boundary located 3-Kb upstream of major alpha-globin enhancer, MRE. Fusion of these small TADs in erythroid cells (both proliferating and differentiated) coincides with the decrease of CTCF binding at the boundary element, which could be a prerequisite for the establishment of spatial contacts between the MRE and downstream alpha-globin gene promoters. However, the loss of CTCF binding at the other TAD boundaries in this region in differentiated HD3 cells does not significantly change the TAD profile, which indicates the existence of CTCF-independent mechanisms of TAD structural maintenance.