text: The Arc system connects the electron transport chain to regulation of certain genes, allowing aerobic respiration to occur in the presence of oxygen and fermentation to take place when no oxygen is present. This is done through the connection of ArcB with quinones from the electron transport chain. Oxidized quionones, from the electron transport chain, act to inhibit autophosphorylation of ArcB during aerobic respiration. This in turn prevents the phosphorylation of ArcA, turning off the activated operons.
  It has been determined that the Arc system regulates as many as 30 genes, with repression of the following examples: cytochrome o oxidase, cytochrome d oxidase, and various gluconeogenic enzymes, such as for the glyoxylate cycle, and fatty acid oxidation. It also induces the expression of Pyruvate formate lyase.
  One of the major genes controlled is the sdh-lacZ operon. This, in part, codes for the synthesis of succinate dehydrogenase, a key element in the TCA cycle. ArcA turns expression of the sdh-lacZ operon off in the presence of oxygen, stopping procession of the TCA cycle. It also activates lctD and pyruvate formate lyase. These genes are critical in the lactic acid fermentation process. They are activated in anoxic conditions to allow the cell to continue producing ATP and growing even in less favorable conditions. ArcA represses many of the other enzymes involved in the TCA cycle as well including flavoprotein dehydrogenases, and ubiquinones oxidases. It also represses enzymes involved in synthesizing glyoxylate, some dehydrognases used in aerobic growth, and enzymes involved in fatty acid oxidation. Activated operons include genes for the pyruvate formate-lyase pathway and enzymes involved in synthesizing cobalamin.
  Another of the genes effected codes for the production of certain cytochromes. ArcA respresses cytochrome bo oxidase and activates cytochrome bd oxidase. Cytochromes are classified based on the hemes they possess, in this case cytochrome bo possesses a heme c while cytochorme bd oxidase psoseses a heme d. Cytochormes bo oxidase is one of the main electron transporters during the elctron transport chain of aerobic respiration. It has the ability to reduce most organic compounds found in cellular metabolism. Cytochrome bd oxidase is activated in anaerobic conditions. It has a higher affinity for oxygen than cytochrome bo oxidase which may be useful to cells in anoxic conditions.