Reactive oxygen species (ROS) including superoxide, hydrogen peroxide, and hydroxyl radicals occur naturally as a byproduct of aerobic respiration. To mitigate damages caused by ROS, Escherichia coli employs defenses including two cytosolic superoxide dismutases (SODs), which convert superoxide to hydrogen peroxide. Deletion of both sodA and sodB, the genes coding for the cytosolic SOD enzymes, results in a strain that is unable to grow on minimal medium without amino acid supplementation. Additionally, deletion of both cytosolic SOD enzymes in a background containing the relA1 allele, an inactive version of the relA gene that contributes to activation of stringent response by amino acid starvation, results in a strain that is unable to grow aerobically, even on rich medium. These observations point to a relationship between the stringent response and oxidative stress. To gain insight into this relationship, suppressors were isolated by growing the ∆sodAB relA1 cells aerobically on rich medium, and seven suppressors were further examined to characterize distinct colony sizes and temperature sensitivity phenotypes. In three of these suppressor-containing strains, the relA1 allele was successfully replaced by the wild type relA allele to allow further study in aerobic conditions. None of those three suppressors were found to increase tolerance to exogenous superoxides produced by paraquat, which shows that these mutations only overcome the superoxide buildup that naturally occurs from deletion of SODs. Because each of these suppressors had unique phenotypes, it is likely that they confer tolerance to SOD-dependent superoxide buildup by different mechanisms. Two of these three suppressors have been sent for whole-genome sequencing to identify the location of the suppressor mutation and determine the mechanism by which they confer superoxide tolerance.

Peptidyl-prolyl cis-trans isomerases (PPIases) are a class of chaperone proteins that catalyze isomerization between the cis and trans configuration of a peptide bond. Flavobacterium johnsoniae, a model organism for the study of bacterial gliding motility and the Type 9 Secretion System (T9SS), has six different proteins that are predicted to encode PPIases. Loss of one putative PPIase (GldI) results in a complete lack of gliding motility. However, loss of another putative PPIase encoded by fjoh_4997 has no impact on motility. In this study, genes fjoh_2367 and fjoh_2368, which are immediately downstream of gldI and have protein products homologous to GldI, were deleted. We found that the mutants exhibited a decrease in speed as compared with the wild type (1.62 ± 0.77 μm/s for mutant 5 and 1.60 ± 0.67 μm/s for mutant 6 vs 2.02 ± 0.62 μm/s for wild type). The slope of the mean square displacement of single cells was lower for mutants as compared with the wild type (1.08 for mutant 5, and 1.13 for mutant 6 vs 1.48 for wild type). Additionally, mutant colonies exhibited less spreading on PY2 agar plates than wild type, as shown by their measured colony diameters, which were 22.91 ± 2.18 mm for wild type, 16.75 ± 2.17 mm for mutant 5, and 15.97 ± 1.12 mm for mutant 6.