DIFFERENTIAL GENE EXPRESSIONS OF SUPEROXIDE DISMUTASE ISOFORMS IN THE LIVERWORT, MARCHANTIA PALEACEA VAR. DIP TERA

Superoxide dismutase (SOD) catalyzes the first step in the scavenging system of active oxygens by the disproportionation of superoxide anion radicals to hydrogen peroxide and molecular oxygen. The liverwort, Marchantia paleacea var. diptera contains three SOD isoforms： CuZn-SOD in cytosol, Mn-SOD in mitochondoria and Fe-SOD in chloroplast. In this study, we isolated cDNA clones encoding a Fe-SOD and a Mn-SOD from the liverwort. The deduced amino acid sequences are similar to those of higher plant SOD sequences. Genomic Southern blot analysis indicated that Mn-SOD gene and Fe-SOD gene exist as a single copy gene. Using these cDNA clones, together with previously isolated cDNA encoding the CuZn-SOD as a probe in Northern blot analysis, we studied the level of transcripts in cells grown under different growth conditions. The mRNA level of Mn-SOD gene was higher in heterotrophic cells than in photoautotrophic cells, and that of Fe-SOD gene was higher in heterotrophic cells grown in the light than in the dark. These results suggest that the gene expression of Mn-SOD is regulated by sugar while that of Fe-SOD by light. On transfer of photoautotrophic cells to darkness, the mRNA level of Fe-SOD gene was reduced while that for CuZn-SOD was increased. Addition of photosynthetic electron transport inhibitor DCMU increased the mRNA level of CuZn-SOD in the light, but did not affect that of Fe-SOD. These results suggest that different mechanisms are involved in regulation of the gene expressions of three SOD isoforms in the liverwort.