Rhodospirillum rubrum
Rhodospirillum rubrum ("R. rubrum") is a Gram-negative, pink-coloured Proteobacterium, with a size of 800 to 1000 nanometers. It is a facultative anaerobe, thus capable of using oxygen for aerobic respiration under aerobic conditions, or an alternative terminal electron acceptor for anaerobic respiration under anaerobic conditions. Alternative terminal electron acceptors for "R. rubrum" include dimethyl sulfoxide or trimethylamine oxide. Under aerobic growth photosynthesis is genetically suppressed and "R. rubrum" is then colorless. After the exhaustion of oxygen, "R. rubrum" immediately starts the production of photosynthesis apparatus including membrane proteins, bacteriochlorophylls and carotenoids, i.e. the bacterium becomes photosynthesis  active. The repression mechanism for the photosynthesis is poorly understood. The photosynthesis of "R. rubrum" differs from that of plants as it possesses not chlorophyll a, but bacteriochlorophylls. While bacteriochlorophyll can absorb light up to a maximum wavelength of 800 to 925 nm, chlorophyll absorbs light having a maximum wavelength of 660 to 680 nm. "R. rubrum" is a spiral-shaped bacterium (spirillum, plural form: spirilla). "R. rubrum" is also a nitrogen fixing bacterium, i.e., it can express and regulate nitrogenase, a protein complex that can catalyse the conversion of atmospheric dinitrogen into ammonia. When the bacteria are exposed to  ammonia, darkness, and phenazine methosulfate, nitrogen fixation stops. Due to this important property, "R. rubrum" has been the test subject of many different groups, so as to understand the complex regulatory schemes required for this reaction to occur. It was in "R. rubrum" that, for the first time, post-translational regulation of nitrogenase was demonstrated. Nitrogenase is modified by an ADP-ribosylation in the arginine residue 101 (Arg101) in response to the so-called "switch-off" effectors - glutamine or ammonia - and darkness. "R. rubrum" has several potential uses in biotechnology: