Pm. Dunman et al., Escherichia coli cells defective for the recN gene display constitutive elevation of mutagenesis at 3,N-4-ethenocytosine via an SOS-induced mechanism, MOL MICROB, 37(3), 2000, pp. 680-686
The Escherichia coli UVM (UV Modulation of mutagenesis) response is a DNA d
amage-inducible mutagenic pathway detected as significantly increased mutag
enesis at 3,N-4-ethenocytosine (epsilon C) lesions borne on transfected sin
gle-stranded M13 vector DNA, All major classes of DNA-damaging agents can i
nduce UVM, and the phenomenon is independent of previously characterized mu
tagenic responses in E. coli, To understand this phenomenon further, we set
out to identify and characterize mutants in the UVM response. Screening a
mutant bank of cells defective for 1-methyl-3-nitro-1-nitrosoguanidine-indu
cible genes revealed that defects in the recN gene cause a constitutive ele
vation of mutagenesis at epsilon C residues, In contrast to normal cells th
at show approximate to 6% mutagenesis at epsilon C lesions, but approximate
to 60% upon UVM induction, recN-defective strains display approximately 50
% mutagenesis at epsilon C lesion sites in untreated cells, However, the re
cN-mediated mutagenesis response was found to require the recA gene and the
umuDC genes, and could be suppressed in the presence of a plasmid harbouri
ng the SOS transcriptional repressor LexA. These results imply that recN ce
lls are constitutively active for SOS mutagenesis functions. The observatio
n that epsilon C mutagenesis is enhanced in recN cells confirms previous fi
ndings that mutagenesis at epsilon C can also be independently elevated by
the SOS pathway.