Isolation and characterisation of a full-length genomic clone encoding a plastidic glucose 6-phosphate dehydrogenase from Nicotiana tabacum

We describe here the isolation and characterisation of the first full-lengt h genomic clone encoding a plant glucose 6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) from Nicotiana tabacum L. cv Samsun. The gene was expressed in all tissues, including roots, leaves, stems and flowers. Comparison of the gene with other known plant G6PDH cDNAs grouped this sequence with plastid ic isoforms. The protein, minus a putative plastidic transit sequence, was overexpressed in Escherichia coli as a glutathione S-transferase fusion pro tein. The resulting protein was shown to be immunologically related to the potato plastidic G6PDH. This suggests that the sequence described here code s for a plastidic isoform. Plastidic G6PDH mRNA was induced in both roots a nd leaves in response to KNO3 and the induction in roots was approximately 4 times the response seen in leaves. Sequence analysis of the 5'-untranslat ed region of the genomic clone indicated the presence of several NIT2 eleme nts, which may contribute to the control of the expression of this gene. Pl astidic G6PDH mRNA levels did not appear to respond to light.