Directed mutagenesis shows that the preceding region of the chloroplast fructose-1,6-bisphosphatase regulatory sequence is the thioredoxin docking site.

The alignment of the six higher plant photosynthetic fructose-1,6-bisphosphatases (FBPases) so far sequenced shows a lack of homology in the region which just precedes the cluster engaged in light modulation. Earlier experiments suggested that this region is the docking point in FBPase-thioredoxin (Trx) binding, and could be responsible for the interspecific differences in the enzyme-Trx interaction and Trx ability for FBPase activation. Using a pea chloroplast FBPase-coding cDNA, we have prepared two chimeric clones for FBPase. One of them (pDELFBP) shows a deletion of the 17 amino acids (Leu154 to Glu170) coding sequence, whereas in the second (pPFBPW) the above sequence was substituted by the corresponding one of the wheat enzyme. After Escherichia coli overexpression in pET-3d and later purification, both modified FBPases showed FBPase activity when determined under non-reducing conditions. However, only DELFBP lost the Trx f modulatory effect, indicating the important role played by this fragment in FBPase-Trx interaction and activity. Under these conditions the substituted PFBPW enzyme retains FBPase activity, even though clearly diminished. Superose 12 filtration experiments after preincubating the wild-type and modified FBPases with Trx f, showed the existence of an enzyme-Trx f binding with the wild-type and the substituted PFBPW, but not with the deleted DELFBP protein. Similarly, gradient PAGE under native conditions, followed by Western blot and developing with FBPase and Trx f antibodies, indicated the existence of such a binding between the wild-type and PFBPW, on the one hand, and both Trxs f and m, on the other, although never with the deleted DELFBP enzyme. These results show the central role played by the regulatory site preceding fragment of chloroplast FBPase in its binding with Trx. Computer-aided tridimensional models for the wild-type and modified FBPases are proposed.