CLONING AND CHARACTERIZATION OF A NUCLEAR GENE ENCODING A STARCH-BRANCHING ENZYME FROM THE MARINE RED ALGA GRACILARIA-GRACILIS

The biosynthesis of starch in red algae occurs in the cytosol, in cont rast to green plants where it takes place in the plastid. We have clon ed a nuclear gene from the red alga Gracilaria gracilis that encodes a homolog of starch-branching enzymes (SBEs); this gene, which is appar ently intron-free, was designated as GgSBE1. A potential TATA box, CAA T boxes, and other potential regula tory elements were observed in its 5' flanking region. The encoded 766-aa peptide shares significant seq uence similarity with SBEs from green plants (at least 40%), and with glycogen-branching enzymes (GBEs) from human (46%) and Saccharomyces c erevisiae (45%). Southern-hybridization analysis indicates that the ge ne is single-copy, although weaker signals suggest that related genes exist in the genome of G. gracilis. Phylogenetic analyses indicate tha t GgSBE1 groups within the eukaryote branching enzymes (BEs) and not w ith eubacterial GBEs, suggesting that its gene has not been derived di rectly from an endosymbiotic cyanobacterium, but instead is ancestrall y eukaryotic.