Multicopy Suppressors of the Cold-sensitive Phenotype of the PcsA68 (dinD68) Mutation in Escherichia Coli

The Escherichia coli strain cs2-68 is a cold-sensitive (c) mutant that forms a long filamentous cell at 20 degrees C with a large nucleoid mass in its central region. We have recently shown that the pcsA68 mutation causing the cs phenotype is a single-base substitution within the dinD gene, a DNA damage-inducible gene which maps at 82 min. Since null mutants of the pcsA (dinD) gene are viable, with no discernible defect in cell growth, the cs phenotype is attributed to a toxic effect by the mutant protein. In an attempt to identify a target(s) for the toxic pcsA68 mutant protein, we screened for chromosomal fragments on multicopy plasmids that could suppress the cs phenotype. Three different BamHI fragments were found to suppress cold sensitivity, and the lexA, dinG, and dinI genes were identified to be responsible for the suppression in each fragment. DinG shares multiple motifs with many DNA helicases. The complete sequence of dinI revealed that DinI is a small protein of 81 amino acids. It is similar in size and sequence to ImpC of the Salmonella typhimurium plasmid TP110 and to a protein (ORFfs) of the retronphage phi R67, both of which are also under the control of LexA.