Divergent Roles of RelA and c-Rel in Establishing Chromosomal Loops upon Activation of the Igκ Gene

Precise regulation of eukaryotic gene expression requires interactions between distal cis-acting regulatory sequences with the looping out of the intervening DNA, but how trans-acting regulatory proteins work to establish and maintain DNA loops during gene activation remains largely unexplored. LPS-induced transcription of the mouse Igkappa gene in B lymphocytes utilizes three distal enhancers and requires the transcription factor NF-kappaB, whose family members include RelA and c-Rel. Using chromosome conformation capture technology in combination with chromatin immunoprecipitation, here we demonstrate that LPS-induced Igkappa gene activation creates chromosomal loops by bridging together all three pairwise interactions between the distal enhancers and RNA polymerase II, the apparent molecular tie for the bases of these loops. RelA and actin polymerization are essential for triggering these processes, which do not require new transcription, protein synthesis, or c-Rel. We have thus identified both essential and nonessential events that establish higher order chromatin reorganization during Igkappa gene activation.