served when N^^H4+ was administered. Sissakian (29) has 

 reviewed evidence that protein can be synthesized in isolated 

 chloroplasts from nonprotein nitrogen, including peptides. 



In experiments in this laboratory (30) it recently has 

 been possible to measure the proportion of the total carbon 

 fixed by Chlorella pyrenoidosa, which is directly incorporated 

 into certain key amino acids. These experiments show that, 

 during steady-state photosynthesis in bright light with an 

 adequate supply of inorganic nutrients, the synthesis of these 

 amino acids can account for 60 per cent of all the carbon 

 fixed by the algae and 30 per cent of the uptake of NH4+, 

 which is also measured. If the light is turned off, the NH4 + 

 uptake and C^* fixation into amino acids are both accelerated 

 for about 10 minutes and then drop to a very small fraction 

 of the rates in the light. Finally, these experiments indicate 

 clearly that in Chlorella pyrenoidosa there are at least two 

 pools of alanine, glutamic acid, aspartic acid, and serine, and 

 probably other amino acids as well. One of these pools, 

 especially in the cases of alanine and aspartic acid, is labeled 

 extremely rapidly after the introduction of C^*02 to the 

 algae. So rapidly are these compounds labeled, in fact, that 

 the site of their synthesis must be freely accessible to their 

 photosynthetically formed precursors, namely, phosphoenol- 

 pyruvic acid and PGA (see Figure 1). The studies of Tolbert 

 (22) and Moses et al. (31) indicate that the photosynthetic 

 pools are isolated from the extrachloroplastic region. We 

 conclude, therefore, that in Chlorella the more rapidly 

 labeled pools of amino acids are located at the site of photo- 

 synthetic carbon reduction, probably in the chloroplast. 



The rates of flow of carbon through these pools of amino 

 acids as determined from kinetic labeling data with Chlorella 

 in a typical experiment are shown in Table 1. 



The amino acids shown in Table 1 are those most prom- 

 inently labeled with C^^ during a few minutes of photosyn- 

 thesis. In addition, a nuinber of other amino acids become 

 labeled as time passes. The rates of labeling seem to indicate 

 that the carbon skeletons of these other acids are probably 



30 



