196 METABOLISM AND PHYSIOLOGY 



Frenkel's (1) discovery in 1954 of light-induced phosphorylation in 

 extracts from R. rubrurn led to the immediate question: is the process 

 linked to electron transport? Experimental support for this possibility 

 was first reported by Smith and M, Baltscheffsky (2,3), who demon- 

 strated that low concentrations of 2-n-heptyl-4-hydroxyquinoline-N- 

 oxide (HOQNO), which were known to specifically inhibit mitochondrial 

 electron transport, strongly inhibited photophosphorylation, thereby 

 causing an oxidation of cytochrome c^ and a probable reduction of a 

 6-type cytochrome. These findings indicated the following similarity 

 between electron transport reactions in mitochondrial oxidative phos- 

 phorylation and bacterial photophosphorylation: 



HOQNO 



— > 6- type cytochrome 1 ^ c-type cytochrome — *- 



Additional evidence for the participation of cytochrome c^in bacterial 

 photophosphorylation was recently obtained by Horio and K amen (4), 

 and of both cytochrome b and cytochrome c^ by Nishimura (5). 



Another similarity became apparent, when one of us (H. B.) obtained 

 results indicating that a flavoprotein functions as electron carrier in 

 the cyclic photophosphorylation system oiR. riibrum (6). In brief, the 

 evidence was: 1) stimulation of photophosphorylation by rather high 

 concentrations of FAD, 2) inhibition by rather low concentrations of 

 atebrin and 3) reversal of the atebrin-induced inhibition by rather 

 high concentrations of FAD. It was also shown that both the basal rate 

 of photophosphorylation and the stimulation caused by added FAD were 

 strongly inhibited by HOQNO and antimycin A (Table 1 in reference 6). 

 This eliminated the possibility that a nonphysiological bypass had 

 occurred from the added FAD to cytochrome c^ (Horio and Kamen (7) 

 have recently emphasized that such bypass reactions may occur at 

 high concentrations of FAD). Our data thus suggested resemblance 

 between bacterial photophosphorylation and mitochondrial oxidative 

 phosphorylation at the flavin level, giving experimental support for 

 the occurrence of the following electron transport sequence in both 

 systems: 



HOQNO 



— >-flavoprotein — > 6- type cytochrome 1 ^c-type cytochrome — ► 



The first direct evidence for a photochemical reaction sequence 



c-type cytochrome •►chlorophyll 



was obtained with the photosynthetic bacterium Chromatium by Chance 

 and Nishimura (8), who demonstrated that this reaction is temperature- 

 independent from 80° to 298°K. Recently, Clayton (9) reported that 



