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 contrast to green plants where it takes place in the plastid. We have cloned a nuclear gene from the red alga Gracilaria gracilis that encodes a homolog of starch-branching enzymes (SBEs); this gene, which is apparently intron-free, was designated as GgSBE1. A potential TATA box, CAAT boxes, and other potential regulatory elements were observed in its 5' flanking region. The encoded 766-aa peptide shares significant sequence similarity with SBEs from green plants (at least 40%), and with glycogen-branching enzymes (GBEs) from human (46%) and Saccharomyces cerevisiae (45%). Southern-hybridization analysis indicates that the gene is single-copy, although weaker signals suggest that related genes exist in the genome of G. gracilis. Phylogenetic analyses indicate that GgSBE1 groups within the eukaryote branching enzymes (BEs) and not with eubacterial GBEs, suggesting that its gene has not been derived directly from an endosymbiotic cyanobacterium, but instead is ancestrally eukaryotic.