Occurrence of cytoplasmic f- and m-type thioredoxins in leaves

Abstract

There is a growing body of evidence that thioredoxins (proteins that act as regulatory messengers in linking light to enzyme modulation in chloroplasts [ 1,2]) occur in multiple forms in photosynthetic cells [3-61. Two types of thioredoxins are found in chloroplasts (thioredoxins f and m), and a third type is found outside chloroplasts, possibly in the cytoplasm (thioredoxin c) [6]. Chloroplast thioredoxinsfand m are reduced photochemically by chloroplasts via ferredoxin and ferredoxin-thioredoxin reductase or, independently of light, in vitro by the nonphysiological sulfhydryl reagent dithiothreitol. When reduced, thioredoxinf activates specific chloroplast enzymes, including enzymes of the reductive pentose phosphate cycle (fructose-l ,6-bisphosphatase (Fru-Psse), NADPglyceraldehyde 3-phosphate dehydrogenase, phosphoribulokinase, sedoheptulose-1,7-bisphosphatase [6]) and an enzyme of secondary plant metabolism (phenylalanine ammonia lyase [7]). By contrast, thioredoxin m is specific in its activation of the single chloroplast enzyme, NADP-malate dehydrogenase (NADP-MDH [6]). The function of cytoplasmic thioredoxin c is unknown. In our research, thioredoxin c resembled its chloroplast counterparts in promoting the activation of chloroplast enzymes. However, unlike chloroplast thioredoxins, thioredoxin c appeared to be nonspecific in that when reduced with dithiothreitol it