420 M. D, Kamen and R. G. Bartsch 



in the only readily available species (strain D of the genus Chroinatiwn) 

 which is representative of the purple sulphur bacteria (Thiorhodaceae) 

 (Kamen and Vernon, 1955). Preliminary attempts to detect RHP in other 

 photosynthetic systems, such as green plants, algae and the green sulphur 

 bacteria, have been unsuccessful to the present. If further studies, now in 

 progress, should confirm that the occurrence of this unique haemoprotein is 

 restricted to the purple bacteria, this result would occasion no great surprise. 

 The same condition exists with regard to the chlorophyll and carotenoid 

 components of the purple bacteria, relative to those of other photosynthetic 

 tissues (Van Niel, 1944; Goodwin, 1955; Goodwin and Land, 1956^; 

 Goodwin, 1956; Goodwin, Land and Sissins, 1956; Goodwin and Land, 

 1956ft). 



It is possible that a haematin compound similar to RHP is present in a 

 halotolerant nitrate-reducing coccus studied by Taniguchi, Asano, lida, 

 Kono, Ohmachi and Egami (1958). A soluble fraction obtained from cell 

 suspensions of this micro-organism contains an enzyme active as a hydroxyl- 

 amine reductase, which exhibits absorption spectra and chemical properties 

 closely similar to those of RHP (see discussion in a later section). 



The fact that RHP occurs in the purple photosynthetic bacteria, and 

 possibly in the nitrate-reducing bacterium mentioned above, points to the 

 desirability of searching for RHP among bacterial species which are chemo- 

 synthetic analogues of the photosynthetic bacteria, e.g. sulphate-reducers, 

 sulphur oxidizers, ammonia and hydrogen oxidizers, nitrate-reducers, etc. 



Many studies on the absorption properties of bacterial suspensions using 

 the elegant methods of dynamic spectrophotometry developed by Chance 

 (1954), Duysens (1954) and others have revealed the existence of spectro- 

 scopic entities among a variety of bacterial species (Bacillus subtilis, Proteus 

 vulgaris, Achromobacter fischeri. Staphylococcus albus, etc.) which appear to 

 resemble RHP closely (Smith, 1954). These pigments appear to function as 

 terminal oxidases, and the term, 'cytochrome o\ has been suggested to denote 

 them as a class (Castor and Chance, 1959). In the particular case of R. 

 rubrum, isolated RHP and the cytochrome o moiety are so much alike 

 spectroscopically, there is strong temptation to equate them and to consider 

 RHP as a prototype for the cytochrome o class. We shall return to this point 

 later. 



Physical Properties 



RHP has been isolated in varying degrees of purity from all the purple 

 photosynthetic bacteria, but in only two cases has it been obtained sufficiently 

 pure and in such amounts as to permit extensive investigation of its physical 

 and chemical properties. These are the original RHP of R. rubrum and that 

 of the strict photoanaerobe, Chromatium (strain D). Salient physical 

 properties of these two preparations are listed in Table 1. The references 



