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113 V(D)J recombination targeted in cis by transcription induced DNA supercoiling

models of PCNAK107-Ub, PCNAK164-Ub and PCNAK164SUMO complexes consistent with solution small angle X-ray scattering (SAXS) data. We show that Ub and SUMO have distinct modes of association to PCNA. Ubiquitin adopts discrete docked binding conformations and the position of ubiquitin attachment, 107 vs. 164, alters conformation. By contrast, SUMO associates by simple tethering and adopts extended flexible conformations. These structural differences are the result of the opposite electrostatic potentials of SUMO and Ub. The unexpected contrast in conformational behavior of Ub-PCNA and SUMO-PCNA has implications for interactions with partner proteins, interacting surfaces accessibility, and access points for pathway regulation.