THE MECHANISM OF THE AUXOTROPHY FOR SULFUR-CONTAINING AMINO-ACIDS IMPOSED UPON ESCHERICHIA-COLI BY SUPEROXIDE

Defects in both of the genes coding for the cytosolic superoxide dismu tases (SODs) of Escherichia coli impose an oxygen-dependent nutritiona l requirement for cysteine. This is now seen to be a bradytrophy, rath er than an absolute auxotrophy, since lack of Cys merely imposed a gro wth lag and escape from this growth lag did not involve genetic revers ion. This Cys bradytrophy was not seen in the SOD-competent parental s train, and it was relieved by a cell-permeant mimic of SOD activity; h ence, it was due to O-2(radical anion). It was also relieved by an osm olyte, such as sucrose; hence, it appears due to leakage from the cell of some component needed for Cys biosynthesis. Medium conditioned by the aerobic growth of the SOD-defective strain relieved the growth lag . Bioassays with Cys mutants suggested that the conditioned medium con tained SO33- or its equivalent, and sulfite per se was able to elimina te the growth lag. However, some component of the conditioned medium r eacted with added sulfite and interfered with attempts to assay for it colorimetrically. These results suggest that the cell envelope of the SOD-defective strain was weakened, directly or indirectly, by O-2(rad ical anion) and then leaked sulfite. This prevents cysteine biosynthes is until sulfite accumulates in the medium.