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Journal of Bacteriology, November 1999, p. 7005-7013, Vol. 181, No. 22
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

The Bacteriophage P1 HumD Protein Is a Functional Homolog of the Prokaryotic UmuD'-Like Proteins and Facilitates SOS Mutagenesis in Escherichia coli

Mary P. McLenigan,1 Olga I. Kulaeva,1,dagger Don G. Ennis,2 Arthur S. Levine,1,Dagger and Roger Woodgate1,*

Section on DNA Replication, Repair and Mutagenesis, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-2725,1 and Department of Biology, University of Louisiana, Lafayette, Louisiana 705042

Received 9 June 1999/Accepted 26 August 1999

The Escherichia coli umuD and umuC genes comprise an operon and encode proteins that are involved in the mutagenic bypass of normally replication-inhibiting DNA lesions. UmuD is, however, unable to function in this process until it undergoes a RecA-mediated cleavage reaction to generate UmuD'. Many homologs of umuDC have now been identified. Most are located on bacterial chromosomes or on broad-host-range R plasmids. One such putative homolog, humD (homolog of umuD) is, however, found on the bacteriophage P1 genome. Interestingly, humD differs from other umuD homologs in that it encodes a protein 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 mutagenesis 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 mutagenesis and increased UV survival to normally nonmutable recA430 lexA(Def) and UV-sensitive Delta umuDC recA718 lexA(Def) strains, respectively. Complementing activity was reduced when HumD was expressed from a low-copy-number plasmid, but this observation is explained by immunoanalysis which indicates that HumD is normally poorly expressed in vivo. In vitro analysis revealed that like UmuD', HumD forms a stable dimer in solution and is able to interact with E. coli UmuC and RecA nucleoprotein filaments. We conclude, therefore, that bacteriophage P1 HumD is a functional homolog of the UmuD'-like proteins, and we speculate as to the reasons why P1 might require the activity of such a protein in vivo.

* Corresponding author. Mailing address: Building 6, Room 1A13, NICHD, NIH, 9000 Rockville Pike, Bethesda, MD 20892-2725. Phone: (301) 496-6175. Fax: (301) 594-1135. E-mail: woodgate{at}helix.nih.gov.

dagger Present address: Molecular Biology Research Program, Henry Ford Hospital, Detroit, MI 48202.

Dagger Present address: School of Medicine, University of Pittsburgh, Pittsburgh, PA 15261.

Journal of Bacteriology, November 1999, p. 7005-7013, Vol. 181, No. 22
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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