Substitution of UmuD′ for UmuD does not affect SOS mutagenesis

Abstract

In order to study the role of UmuDC proteins in SOS mutagenesis, we have constructed new Escherichia coli K-12 strains to avoid i) over-production of Umu proteins, ii) the formation of unwanted mixed plasmid and chromosomal Umu proteins upon complementation. We inserted a mini-kan transposon into the umuD gene carried on a plasmid. The insertion at codon 24 ends protein translation and has a polar effect on the expression of the downstream umuC gene. We transferred umuD24 mutation to the E coli chromosome. In parallel, we subcloned umuD + umuC + or umuD′ umuC + genes into pSC101, a low copy number plasmid. In a host with the chromosomal umuD24 mutation, plasmids umuD + umuC + or umuD′ umuC + produced elevated resistance to UV light and increased SOS mutagenesis related to a gene dosage of about 3. UV mutagenesis was as high in umuD′ umuC + hosts devoid of UmuD + protein as in umuD + umuC + hosts. UmuD′ protein, the maturated form of UmuD, can substitute for UmuD in SOS mutagenesis.