The expression of Escherichia coli umuD gene products is upregulated as part of the SOS response to DNA damage. UmuD is initially produced as a 139-amino-acid protein, which subsequently cleaves off its N-terminal 24 amino acids in a reaction dependent on RecA/single-stranded DNA, giving UmuD'. The two forms of the umuD gene products play different roles in the cell. UmuD is implicated in a primitive DNA damage checkpoint and prevents DNA polymerase IV-dependent -1 frameshift mutagenesis, while the cleaved form facilitates UmuC-dependent mutagenesis via formation of DNA polymerase V (UmuD'(2)C). Thus, the cleavage of UmuD is a crucial switch that regulates replication and mutagenesis via numerous protein-protein interactions. A UmuD variant, UmuD3A, which is noncleavable but is a partial biological mimic of the cleaved form UmuD', has been identified. We used hydrogen-deuterium exchange mass spectrometry (HXMS) to probe the conformations of UmuD, UmuD', and UmuD3A. In HXMS experiments, backbone amide hydrogens that are solvent accessible or not involved in hydrogen bonding become labeled with deuterium over time. Our HXMS results reveal that the N-terminal arm of UmuD, which is truncated in the cleaved form UmuD', is dynamic. Residues that are likely to contact the N-terminal arm show more deuterium exchange in UmuD' and UmuD3A than in UmuD. These observations suggest that noncleavable UmuD3A mimics the cleaved form UmuD' because, in both cases, the arms are relatively unbound from the globular domain. Gas-phase hydrogen exchange experiments, which specifically probe the exchange of side-chain hydrogens and are carried out on shorter timescales than solution experiments, show that UmuD' incorporates more deuterium than either UmuD or UmuD3A. This work indicates that these three forms of the UmuD gene products are highly flexible, which is of critical importance for their many protein interactions.