STA11, a Chlamydomonas reinhardtii Locus Required for Normal Starch Granule Biogenesis, Encodes Disproportionating Enzyme. Further Evidence for a Function of α-1,4 Glucanotransferases during Starch Granule Biosynthesis in Green Algae

In Chlamydomonas reinhardtii, the presence of a defective STA11 locus results in significantly reduced granular starch deposition displaying major modifications in shape and structure. This defect simultaneously leads to the accumulation of linear malto-oligosaccharides (MOS). The mutants of STA11 were showed to lack D-enzyme, a plant alpha-1,4 glucanotransferase analogous to the Escherichia coli amylomaltase. We have cloned and characterized both the cDNA and gDNA corresponding to the C. reinhardtii D-enzyme. We now report allele-specific modifications of the D-enzyme gene in the mutants of STA11. These allele-specific modifications cosegregate with the corresponding sta11 mutations, thereby demonstrating that STA11 encodes D-enzyme. MOS production and starch accumulation were investigated during day and night cycles in wild-type and mutant C. reinhardtii cells. We demonstrate that in the algae MOS are produced during starch biosynthesis and degraded during the phases of net polysaccharide catabolism.