Regulation of SOS mutagenesis by proteolysis.
DNA damage-inducible mutagenesis in Escherichia coli is largely dependent upon the activity of the UmuD (UmuD') and UmuC proteins. The intracellular level of these proteins is tightly regulated at both the transcriptional and the posttranslational levels. Such regulation presumably allows cells to deal with DNA damage via error-free repair pathways before being committed to error-prone pathways. We have recently discovered that as part of this elaborate regulation, both the UmuD and the UmuC proteins are rapidly degraded in vivo. We report here that the enzyme responsible for their degradation is the ATP-dependent serine protease, Lon. In contrast, UmuD' (the posttranslational product and mutagenically active form of UmuD) is degraded at a much reduced rate by Lon, but is instead rapidly degraded by another ATP-dependent protease, ClpXP. Interestingly, UmuD' is rapidly degraded by ClpXP only when it is in a heterodimeric complex with UmuD. Formation of UmuD/UmuD' heterodimers in preference to UmuD' homodimers therefore targets UmuD' protein for proteolysis. Such a mechanism allows cells to reduce the intracellular levels of the mutagenically active Umu proteins and thereby return to a resting state once error-prone DNA repair has occurred. The apparent half-life of the heterodimeric UmuD/D' complex is greatly increased in the clpX::Kan and clpP::Kan strains and these strains are correspondingly rendered virtually UV non-mutable. We believe that these phenotypes are consistent with the suggestion that while the UmuD/D' heterodimer is mutagenically inactive, it still retains the ability to interact with UmuC, and thereby precludes the formation of the mutagenically active UmuD'2C complex.
Frank
E G
EG
Section on DNA Replication, Repair and Mutagenesis, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-2725, USA.
Ennis
D G
DG
Gonzalez
M
M
Levine
A S
AS
Woodgate
R
R
D000074183:ATPases Associated with Diverse Cellular Activities; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000378:metabolism; D004249:DNA Damage; D004259:DNA-Directed DNA Polymerase; D049071:Endopeptidase Clp; D004926:Escherichia coli / Q000235:genetics* / Q000528:radiation effects; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005838:Genotype; D018832:Molecular Chaperones; D016296:Mutagenesis; D011499:Protein Processing, Post-Translational; D015183:Restriction Mapping; D013014:SOS Response, Genetics; D014158:Transcription, Genetic; D014466:Ultraviolet Rays
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D018832:Molecular Chaperones; C078202:UmuC protein, E coli; D004259:DNA-Directed DNA Polymerase; C055218:UmuD protein, E coli; D049071:Endopeptidase Clp; D000251:Adenosine Triphosphatases; C088072:ClpX protein, E coli; D000074183:ATPases Associated with Diverse Cellular Activities