Project Summary All bacteria must alter their physiology in response to changes in their environment nn order to survive and flourish. One important environmental cue that most species respond to is the presence or absence of molecular oxygen. In most bacterial species, as well as in mitochondria in eukaryotes, oxygen is used during the process of respiration as an energy generating terminal electron acceptor. In its absence, cells must derive energy by alternative means such as fermentation that led to metabolic production of many end products that are of economic importance. For example, fermentation can lead to the production of such commercially important products as acids, alcohols, methane and H2. Even though the use of oxygen by cells is important for life as we know it, oxygen can also have deleterious effects as it can readily form damaging oxygen reactive species such as singlet oxygen. Thus cells must be capable of sensing the presence and absence of oxygen and subsequently alter their metabolism to take advantage of its energy generating potential as well as to survive deleterious effects of this reactive molecule. Often the sensing of oxygen by regulatory proteins is linked to the alteration of cellular physiology by coupling an oxygen-sensing domain with a DNA binding domain that allows direct gene expression control in response to the presence or absence of oxygen. In some photosynthetic species that use light as an alternative energy source, sensing oxygen is also linked to sensing light intensity. In this case there are also a set of photoreceptors that absorb light that subsequently alter gene expression in response to light intensity. This secondary light control is used to ensure that there is proper balance of the generation of reducing power by photosynthesis (electrons) with the utilization of reducing power by respiration that uses oxygen as an electron acceptor. In this proposal, we outline a study of the regulation of bacterial gene expression in response to alterations in oxygen tension and light intensity in the model photosynthetic organism Rhodobacter capsulatus. This organism has a large swing in cellular redox potential caused by the presence of light driven energy production by a photosystem. In addition, these cells can also utilize oxygen and other compounds for respiration. Consequently, this species has a large set of oxygen and light responding transcription factors that are used to alter cellular physiology to promote growth in a variety of conditions with and without the presence of oxygen and/or light. We describe biochemical and genetic studies that will define how several regulatory proteins sense oxygen and light and how they subsequently alter cellular gene expression and metabolism. The results to date show that many oxygen and light responding transcription factors used by Rhodobacter capsulatus are also present in many non-photosynthetic species including important pathogens.