59 



John M. Olson and Robert M. Smillie 



Anacystls the action spectrum of R-j_ vas obtained by cooling the 

 cells to 2" C at \^ich temperature the rate of cytochrome reduc- 

 tion by Rg is negligible. Chlorophyll a vas found to be kO to 70 

 per cent more effective than phycocyanin in R-j^, as vas found by 

 Amesz and Duysens'^/. We also confirmed the finding of Amesz and 

 Duysens that chlorophyll a is also effective in Rg* although much 

 less than is phycocyanin. In Euglena , the action spectrum of Rj^ 

 vas obtained by adding 3-(3,^ dichlorophenyl)-l,l-dimethylurea 

 (DCMU), 1.5 X 10-5 M, to the cell suspension (Fig. 3) in order to 

 block Rg. Ccanparison of the action si)ectrum to the absorption 

 (1-T) spectrum indicates that chlorophyll a is effective in R^^, 

 but that a pigment vith an absorption peak at about 7O5 m^ is 

 considerably more effective. . This pigment probably corresponds 

 to Kok's photocatalyst P-TOOV^^ and Butler's C-705^°^. When the 

 action spectrxnn of cytochrome oxidation (steady-state) is deter- 

 mined in the absence of DCMU, both R^ and Rg determine the curve 

 marked CONTROL in Figure 3. The absorption spectrum of those 

 pigments vhich are effective in both B.-^ and R2 is obtained by 

 subtracting the CONTROL curve from the DCMU ciirve. From Figure 3 

 it can be seen that chlorophyll a is effective in R2 as veil as 

 in Rn . Chlorophyll b apparently does not play a unique role in 

 sensitizing Rp in Euglena, as has been suggested in other instan- 

 ces of green plant photosynthesisC^*"^ . 



Light Reaction No. 1 ; After illumination of Anacystis or 

 Euglena , the cytochrome(s) oxidized in the light return to the 

 reduced state in the dark, even under conditions -vAien Ro is 

 totally inoperative. Certain characteristics of the cytochrome 

 response to R^ suggest that reducing equivalents formed in R^^ can 

 reduce oxidized cytochrome(s) via a temperature sensitive pathvajt 

 The temperature sensitivity of the cytochrcxne reductase activity 

 can be seen from the increase in the half time for the "light- 

 off" cytochrome reduction as the temperature is dropped. In 

 Euglena , for example, t-^/2 increased from ca. 6 sec at 25* to ca. 

 26 sec at 2". The temperature -independence of the "light -on" 

 cytochrome oxidation can be seen from initial rate measurements 

 and from the lover light intensity reqiiired to maintain a given 

 level of cytochrcme oxidation at lover temperature. In Anacystis, 

 for example, the relative intensities of 675 m\x light required to 

 give half maximal cytochrane oxidation at 2°, 20°, and 37° vere 

 1, U, and 10, respectively. Evidence for a cyclic flov of elec- 

 trons via a cytochrome system in R]_ is provided by the kinetics 

 of the "light-on" and "light-off" cytochrome responses at very 

 lov light intensities in both algae. As shovn in Figure k, the 

 oxidation reaction does not become apparent immediately upon 

 illumination vith very lov intensity; a definite lag occurs. The 



