Dimer exchange and cleavage specificity of the DNA damage response protein UmuD

The cellular response to DNA damage in Escherichia coli is controlled in part by the activity of the umuD gene products. The full-length dimeric UmuD2 is the initial product that is expressed shortly after the induction of the SOS response and inhibits bacterial mutagenesis, allowing for error-free repair to occur. Over time, the slow auto-cleavage of UmuD2 to UmuD′2 promotes mutagenesis to ensure cell survival. The intracellular levels of UmuD2 and UmuD′2 are further regulated by degradation in vivo, returning the cell to a non-mutagenic state. To further understand the dynamic regulatory roles of the umuD gene products, we monitored the kinetics of exchange and cleavage of the UmuD2 and UmuD′2 homodimers as well as of the UmuDD′ heterodimer under equilibrium conditions. We found that the heterodimer is the preferred but not exclusive protein form, and that both the heterodimer and homodimers exhibit slow exchange kinetics which is further inhibited in the presence of interacting partner DinB. In addition, the heterodimer efficiently cleaves to form UmuD′2. Together, this work reveals an intricate UmuD lifecycle that involves dimer exchange and cleavage in the regulation of the DNA damage response.

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► UmuD homodimers and the heterodimer exhibit slow exchange kinetics.
► The UmuDD′ heterodimer preferentially forms over the homodimers.
► The UmuDD′ heterodimer preferentially cleaves in trans to form UmuD′.