COPROPORPHYRINOGEN-III OXIDASE FROM BARLEY AND TOBACCO - SEQUENCE-ANALYSIS AND INITIAL EXPRESSION STUDIES

Coproporphyrinogen III oxidase (coprogen oxidase; EC 1.3.3.3) is part of the pathway from 5-aminolevulinate to protoporphyrin IX which is co mmon in all organisms and catalyses oxidative decarboxylation at two t etrapyrrole side chains. We cloned and sequenced full-length cDNAs enc oding coprogen oxidase from barley (Hordeum vulgare L.) and tobacco (N icotiana tabacum L.). They code for precursor peptides of 43.6 kDa and 44.9 kDa, respectively. Import into pea plastids resulted in a proces sed tobacco protein of approx. 39 kDa, which accumulated in the stroma fraction. Induction of synthesis of recombinant putative tobacco matu re coprogen oxidase consisting of 338 amino-acid residues in Escherich ia coli at 20 degrees C result in a catalytically active protein of ap prox. 39 kDa, while induction of its formation at 37 degrees C immedia tely terminated bacterial growth, possibly due to toxic effects on the metabolic balance of tetrapyrrole biosynthesis. The plant coprogen ox idase gene was expressed to different extents in all tissues investiga ted. This is most likely due to the differing requirements for tetrapy rroles in different organs. The steady-state level of mRNA did not sig nificantly differ in etiolated and greening barley leaves. The content of coprogen oxidase RNA reached its maximum in developing cells and d ecreased drastically when cells were completely differentiated. Functi oning of the two photosystems apparatus requires the synthesis of all pigment and protein components during plant development. It is specula ted that the enzymes involved in tetrapyrrole synthesis are developmen tally rather than light-dependently regulated, Regulation of these enz ymes also guarantees a constant flux of metabolic intermediates and av oids photodynamic damage by accumulating porphyrins.