Escherichia coli SOS mutagenesis involves the induction of at least 57 genes in response to DNA damage, including the umuD gene products. UmuD2 is a homodimer of 139-amino acid subunits that interacts with RecA:ssDNA resulting in cleavage of its N-terminal 24-amino acids, yielding UmuD’2. The goal of our research is to determine the conformation and dynamics of the UmuD proteins in order to understand its regulatory role in response to DNA damage. We have discovered an N-terminally truncated version of the UmuD protein that is similar to full-length UmuD2 in cleavage activity, but differs in its conformation. In vivo characterization reveals that this newly identified form is active in facilitating UV mutagenesis and displays a cold sensitive phenotype. This suggests that the length of the arms of the UmuD proteins is critical in regulating function. Indeed, we have shown by EPR that UmuD2 and UmuD’2 arm dynamics are different. Additionally, full-length UmuD2 and cleaved UmuD’2 interact differentially with partner proteins resulting in dramatically different cellular outcomes. Our preliminary work points to yet another level of regulation in which the umuD gene products coordinate the DNA damage response. Research supported by NSF Career MCB-0845033.