THE HEME OXYGENASE GENE (PBSA) IN THE RED ALGA RHODELLA-VIOLACEA IS DISCONTINUOUS AND TRANSCRIPTIONALLY ACTIVATED DURING IRON LIMITATION

Heme oxygenase (HO) catalyzes the opening of the heme ring with the re lease of iron in both plants and animals. In cyanobacteria, red algae, and cryptophyceae, HO is a key enzyme in the synthesis of the chromop horic part of the photosynthetic antennae. In an attempt to study the regulation of this key metabolic step, we cloned and sequenced the pbs A gene encoding this enzyme from the red alga Rhodella violacea. The g ene is located on the chloroplast genome, split into three distant exo ns, and is presumably expressed by a trans-splicing mechanism. The ded uced poly-peptide sequence is homologous to other reported HOs from or ganisms containing phycobilisomes (Porphyra purpurea and Synechocystis sp, strain PCC 6803) and, to a lesser extent, to vertebrate enzymes, The expression is transcriptionally activated under iron deprivation, a stress condition frequently encountered by algae, suggesting a secon d role for HO as an iron-mobilizing agent in photosynthetic organisms.