Abstract
The molecular evolution of SOD exhibits a characteristic distribution of its isozymes and isoforms among organisms in response to oxidative stress. However, the divergence point of chloroplastic (chl) and cytosolic (cyt) CuZn-SOD isozymes is uncertain. We have previously cloned the chl SOD gene from the green alga Spirogyra, and also found two chl SOD isoform genes in the moss Pogonatum and the fern Equisetum. Since recent genome data revealed the presence of chl SOD gene in the prasinophyte alga Ostreococcus, and the occurrence of two chl isoform genes in Physcomitrella, we analyzed the divergence point of chl and cyt SOD and the phylogenetic relationship of chl SOD isoforms in moss and fern. Ostreococcus chl SOD gene showed a high homology with that of Spirogyra, although their exon-intron structures were completely different. The phylogenetic analysis showed a sister-group relationship between chl and cyt SOD genes, suggesting the divergence of both genes from an ancestor before the appearance of streptophyte algae from chlorophytes. Furthermore, it was shown that chl SOD isoform genes in moss and fern were derived from gene duplication after separation of each species.
