Multple Active Forms of UmuD Protein that Regulates Bacterial Mutagenesis

Abstract

The SOS mutagenesis pathway in Escherichia coli involves the induction of at least 57 genes in response to DNA damage, including the umuD gene products. The homodimeric umuD gene products play key roles in regulating the cellular response to DNA damage. UmuD2 is composed of 139-amino acid subunits and is upregulated as part of the SOS response. Subsequently, damage-induced RecA/ssDNA nucleoprotein filaments mediate the slow autocleavage of the N-terminal 24-amino acid arms of UmuD2 yielding UmuD′2. Several models have been proposed wherein the N-terminal arms of UmuD2 are in the cis or trans conformations, with elbows up or down. These conformational variants expose multiple binding surfaces that may interact with other proteins. UmuD2 and UmuD′2 make a number of distinct protein-protein contacts with DNA polymerases and other proteins that both prevent and facilitate potentially mutagenic translesion synthesis. Wild-type UmuD2 and UmuD′2 form exceptionally tight dimers in solution; however, we show that the single amino-acid change N41D generates stable, active UmuD and UmuD′ monomers that functionally mimic the dimeric wild-type proteins. The UmuD N41D monomer is proficient for cleavage and interacts physically with DNA polymerase IV (DinB). Furthermore, the N41D variants facilitate UV-induced mutagenesis and promote overall cell viability. Taken together, these observations show that a monomeric form of UmuD retains substantial function in vivo and in vitro.