Positive Charges Are Important for the SOS Constitutive Phenotype in and Mutants of Escherichia coli K-12.

In Escherichia coli K-12, RecA binds to single-strand DNA (ssDNA) created by DNA damage to form a protein-DNA helical filament that serves to catalyze LexA autoproteolysis, which induces the SOS response. The SOS constitutive (SOS) mutations (E38K) and (Q184K) are both on the outside of the RecA filament, opposite to the face that binds DNA. (E38K) is also able to suppress the UV sensitivity caused by mutations. Both SOS expression and suppression are thought to be due to RecA730's ability to compete better for ssDNA coated with ssDNA-binding protein than the wild type. We tested whether other positively charged residues at these two positions would lead to SOS expression and suppression. We found that 5/6 positively charged residues were SOS and 4/5 of these were also suppressors. While other mutations at these two positions (and others) were suppressors, none were SOS. Three suppressors could be made moderately SOS by adding a operator mutation. We hypothesize two mechanisms for SOS expression: the first suggests that the positive charge at positions 38 and 184 attract negatively charged molecules that block interactions that would destabilize the RecA-DNA filament, and the second involves more stable filaments caused by increases in mutant RecA concentration. In Escherichia coli K-12, SOS constitutive (SOS) mutants of turn on the SOS response in the absence of DNA damage. Some SOS mutants are also able to indirectly suppress the UV sensitivity of mutations. Two SOS mutations, (E38K) and (Q184K), define a surface on the RecA-DNA filament opposite the surface that binds DNA. Both introduce positive charges, and is a suppressor. We tested whether the positive charge at these two positions was required for SOS expression and suppression. We found a high correlation between the positive charge, SOS expression and suppression. We also found several other mutations (different types) that provide suppression but no SOS expression.