Two proteins function in the regulation of photosynthetic CO₂ assimilation in chloroplasts

Abstract

THERE is increasing evidence that products of the light reactions of photosynthesis govern the activity of enzymes involved in CO₂ assimilation by chloroplasts^1–15. Of these products, reductants formed photochemically seem to be of particular importance. Such reductants include the reduced form of ferredoxin^2,6,10, a strongly electronegative chloroplast iron-sulphur protein (E′₀=−0.42 V) that activates the two key chloroplast enzymes fructose 1,6-bis-phosphatase and sedoheptulose l,7-bis-phosphatase. Activation of both of these enzymes requires in addition to reduced ferredoxin a ‘protein factor’ that is indigenous to chloroplasts. In efforts to elucidate the nature of the ferredoxin-linked enzyme activation, we have separated the protein factor into two components¹⁶: (1) a partly purified protein, provisionally named “assimilation regulatory protein a” (ARP_a) and (2) a highly purified chromophore-free protein called “assimilation regulatory protein b” (ARP_b). Only the latter was required for activation when reduced ferredoxin was replaced by the non-physiological sulphydryl reagent dithiothreitol^6,10.