Characterization of a nicotinamide nucleotide transhydrogenase gene from the green alga Acetabularia acetabulum and comparison of its structure with those of the corresponding genes in mouse and Caenorhabditis elegans

Proton-pumping nicotinamide nucleotide transhydrogenase (Nnt) is a membrane -bound enzyme that catalyzes the reversible reduction of NADP(+) by NADH. T his reaction is linked to proton translocation across the membrane. Dependi ng on metabolic conditions, the enzyme may be involved in NADPH generation, e.g., for detoxification of peroxides and/or free radicals and protection from ischemic damage. Nnt exists in most prokaryotes and in animal mitochon dria. It is composed of 2-3 subunits in bacteria and of a single polypeptid e in mitochondria. An open question is whether Nnt exists in any photosynth etic eukaryotes and if so, to which class it belongs. In the present study it is demonstrated that, by cloning and sequencing cDNA and genomic, copies of its NNT gene, an ancient alga, Acetabularia acetabulum (Chlorophyta, Da sycladales), contains a nuclear-encoded Nnt. In contrast to photosynthetic bacteria, this algal Nnt is composed of a single polypeptide of the class f ound in animal mitochondria. Excluding a-poly(A) tail, NNT cDNA from A. ace tabulum is 3688 bp long, consists of eight exons and spans 17 kb. The NNT g ene from mouse was also characterized. Subsequently, the gene organization of the A. acetabulum NNT was compared to those of the homologous mouse (100 kb and 21 exons) and Caenorhabditis elegans (5.1 kb and 18 exons) genes. ( C) 2001 Elsevier Science B.V. All rights reserved.