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Journal of Bacteriology, January 1999, p. 177-185, Vol. 181, No. 1
Department of Biology, Massachusetts
Institute of Technology, Cambridge, Massachusetts
02139,1 and
Department of Biological
Chemistry and Molecular Pharmacology, Harvard Medical School,
Boston, Massachusetts 021152
Received 13 August 1998/Accepted 23 October 1998
The products of the SOS-regulated umuDC operon are
required for most UV and chemical mutagenesis in Escherichia
coli, a process that results from a translesion synthesis
mechanism. The UmuD protein is activated for its role in mutagenesis by
a RecA-facilitated autodigestion that removes the N-terminal 24 amino
acids. A previous genetic screen for nonmutable umuD
mutants had resulted in the isolation of a set of missense mutants that
produced UmuD proteins that were deficient in RecA-mediated cleavage
(J. R. Battista, T. Ohta, T. Nohmi, W. Sun, and G. C. Walker,
Proc. Natl. Acad. Sci. USA 87:7190-7194, 1990). To identify elements
of the UmuD' protein necessary for its role in translesion synthesis,
we began with umuD', a modified form of the
umuD gene that directly encodes the UmuD' protein, and
obtained missense umuD' mutants deficient in UV and methyl
methanesulfonate mutagenesis. The D39G, L40R, and T51I mutations affect
residues located at the UmuD'2 homodimer interface and
interfere with homodimer formation in vivo. The D75A mutation affects a
highly conserved residue located at one end of the central strand in a
three-stranded
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Copyright © 1999, American Society for Microbiology. All rights reserved.
Mutations Affecting the Ability of the
Escherichia coli UmuD' Protein To Participate in SOS
Mutagenesis

-sheet and appears to interfere with
UmuD'2 homodimer formation indirectly by affecting the
structure of the UmuD' monomer. When expressed from a multicopy plasmid, the L40R umuD' mutant gene exhibited a dominant
negative effect on a chromosomal umuD+ gene
with respect to UV mutagenesis, suggesting that the mutation has an
effect on UmuD' function that goes beyond its impairment of homodimer
formation. The G129D mutation affects a highly conserved residue that lies at the end of the long C-terminal
-strand and results in a mutant UmuD' protein that exhibits a strongly dominant negative effect on UV mutagenesis in a umuD+
strain. The A30V and E35K mutations alter residues in the N-terminal arms of the UmuD'2 homodimer, which are mobile in solution.
*
Corresponding author. Mailing address: 68-633, Department of Biology, Massachusetts Institute of Technology,
Cambridge, MA 02139. Phone: (617) 253-6716. Fax: (617) 253-2643. E-mail: gwalker{at}mit.edu.
Present address: School of Life Science, Tokyo University of
Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-03, Japan.
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