STARCHES FROM A TO C - CHLAMYDOMONAS-REINHARDTII AS A MODEL MICROBIALSYSTEM TO INVESTIGATE THE BIOSYNTHESIS OF THE PLANT AMYLOPECTIN CRYSTAL

Wide-angle powder x-ray diffraction analysis was carried out on starch extracted from wild-type and mutant Chlamydomonas reinhardtii cells. Strains containing no defective starch synthases as well as mutants ca rrying a disrupted granule-bound starch synthase structural gene displ ayed the A type of diffraction pattern with a high degree of crystalli nity. Mutants carrying a defect for the major soluble starch synthase (SSS), SSS II, were characterized by a switch to the B type of diffrac tion pattern with very low crystallinity. Mutant strains carrying SSS I as the only glucan elongation enzyme regained some of their crystall inity but switched to the C type of diffraction pattern. Differential scanning calorimetry analysis correlated tightly with the x-ray diffra ction results. Together with the electron microscopy analyses, these r esults establish C. reinhardtii as a microbial model system displaying all aspects of cereal starch synthesis and structure. We further show that SSS II is the major enzyme involved in the synthesis of crystall ine structures in starch and demonstrate that SSS I alone builds a new type of amylopectin structure.