Distinctive properties of bacilliform photosynthetic heliobacteria

Heliobacterium chlorum and Heliobacillus mobilis are closely related N2-fixing anoxyphoto-trophs that contain bacteriochlorophyll g (Bchl g) as the major photopigment. In the presence of O2 and light, the absorbance peak in the infra-red (788 nm) of this novel photoreceptor disappears and absorbance at 670 nm increases simultaneously. These optical changes appear to be due to a photoisomerization reaction which converts Bchl g to a form of green-plant chlorophyll a (in which farnesol replaces phytol). In addition to this unusual property, the Gram-negative heliobacteria present biochemical features (16S RNA base sequence and peptidoglycan structure) indicating an evolutionary relationship with some Gram-positive bacteria. In comparison to H. chlorum, H. mobilis grows more robustly and shows a much lower tendency to spheroplast and lyse; accordingly, H. mobilis is better suited for further investigations on the biology and biochemistry of these exceptional prokaryotes.