Bacterial Adaptive Responses - Coping with Oxygen Limitation, Reactive Nitrogen Species and Disulfide Stress

Abstract: Bacteria have a remarkable ability to adapt to different environments. When bacteria encounter stress that causes macromolecular damage in the cell, leading to a suboptimal performance of cell metabolism, they can elicit an adaptive response. In this thesis Bacillus subtilis is used as the primary model organism to study bacterial adaptation to oxygen limitation, reactive nitrogen species and disulfide stress. Sudden changes in the oxygen availability leads to changes in the intracellular NAD:NADH ratio. In this work, the B. subtilis Rex protein is characterized. Rex acts as a transcriptional repressor that can sense the intracellular NADH concentration. When the concentration of NADH is low, Rex represses genes needed for growth under conditions of low oxygen availability. Reactive nitrogen species (RNS) are present in many environments inhabited by bacteria, and may cause severe damage to bacterial cells. Here, we show that the major enzyme required for protection against RNS in B. subtilis is the Hmp flavohemoglobin. The hmp gene is regulated by the ResDE two-component system and the NsrR transcriptional repressor. Under normal conditions, the bacterial cell cytoplasm is a reducing environment where protein cysteines are kept in their reduced form. However, if the cytoplasm becomes oxidizing, unwanted disulfide bonds may form, a phenomenon known as disulfide stress. Spx is a global regulator governing the disulfide stress response in B. subtilis. Under non-stress conditions, Spx is rapidly degraded by the ClpXP protease. In this work, YjbH is identified as a negative effector of Spx, that is required for the efficient degradation of Spx by ClpXP.