The bacteriophage P1 HumD protein is a functional homolog of the prokaryotic UmuD '-like proteins and facilitates SOS mutagenesis in Escherichia coli

The Escherichia coli umuD and umuC genes comprise an operon and encode prot eins that are involved in the mutagenic bypass of normally replication-inhi biting DNA lesions. UmuD is, however, unable to function in this process un til it undergoes a RecA-mediated cleavage reaction to generate UmnD'. Many homologs of umuDC have now been identified. Mast are located on bacterial c hromosomes or on broad-host-range R plasmids. One such putative homolog, hu mD (homolog of umuD) is, however, found on the bacteriophage P1 genome. Int erestingly, humD differs from other umuD homologs in that it encodes a prot ein similar in size to the posttranslationally generated UmuD' protein and not UmuD, nor is it in an operon with a cognate umuC partner. To determine if HumD is, in fact, a bona fide homolog of the prokaryotic UmuD'-like muta genesis proteins, we have analyzed the ability of HumD to complement UmuD' functions in vivo as well as examined HumD's physical properties in vitro. When expressed from a high-copy-number plasmid, HumD restored cellular muta genesis and increased UV survival to normally nonmutable recA430 lexA(Def) and UV-sensitive Delta umuDC recA718 lexA(Def) strains, respectively. Compl ementing activity was reduced when HumD was expressed from a low-copy-numbe r plasmid, but this observation is explained by immunoanalysis which indica tes that HumD is normally poorly expressed in vivo, In vitro analysis revea led that like UmuD', HumD forms a stable dimer in solution and is able to i nteract with E. coli UmuC and RecA nucleoprotein filaments. We conclude, th erefore, that bacteriophage P1 HumD is a functional homolog of the UmuD'-li ke proteins, and we speculate as to the reasons why P1 might require the ac tivity of such a protein in vivo.