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J. Bacteriol., Jun 1996, 3550-3556, Vol 178, No. 12
EG Frank, M Gonzalez, DG Ennis, AS Levine and R Woodgate
The Escherichia coli Umu proteins play critical roles in damage- inducible
SOS mutagenesis. To avoid any gratuitous mutagenesis, the activity of the
Umu proteins is normally kept to a minimum by tight transcriptional and
posttranslational regulation. We have, however, previously observed that
compared with an isogenic recA+ strain, the steady-state levels of the Umu
proteins are elevated in a recA730 background (R. Woodgate and D. G. Ennis,
Mol. Gen. Genet. 229:10-16, 1991). We have investigated this phenomenon
further and find that another coprotease-constitutive (recA*) mutant, a
recA432 strain, exhibits a similar phenotype. Analysis revealed that the
increased steady-state levels of the Umu proteins in the recA* strains do
indeed reflect an in vivo stabilization of the proteins. We have
investigated the basis for the phenomenon and find that the mutant RecA*
protein stabilizes the Umu proteins by not only converting the labile UmuD
protein to the much more stable (and mutagenically active) UmuD' protein
but by directly stabilizing UmuD' itself. In contrast, UmuC does not appear
to be directly stabilized by RecA* but is instead dramatically stabilized
in the presence of UmuD'. On the basis of these observations, we suggest
that formation of a UmuD'C-RecA*-DNA quaternary complex protects the UmuD'C
proteins from proteolytic degradation and as a consequence helps to promote
the switch from error- free to error-prone mechanisms of DNA repair.
Copyright © 1996, American Society for Microbiology
In vivo stability of the Umu mutagenesis proteins: a major role for RecA
Section on DNA Replication, Repair and Mutagenesis, National Institute of Child Health and Human Development, Bethesda, Maryland 20892-2725, USA.
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