||Genome sequence information of Synechocystis sp. PCC 6803 showed the presence of a putative glycosyltransferase gene (ORF slr0813) that is highly homologous to mammalian ceramide glucosyltransferase. I cloned and expressed the coding region of the algal gene in E. coli and showed the gene product does not perform the analogous function of transferring glucose from UDPG to ceramide as exhibited by mammalian ceramide glucosyltransferase. Since I detected that the algal gene is expressed, I generated a mutant strain of Synechocystis sp. PCC 6803 with the coding region of this putative glycosyltransferase gene (pgt gene) completely knocked out, Synechocystis sp. PCC 6803 ZH24K, and another mutant strain with the pgt gene over expressed by a native promoter, Synechocystis sp. PCC 6803 ZHKPP to elucidate the functional role of this gene in algal cell. Compare with the wild type cells, mutants strains differ in cell size, cell surface structure and hydrophobicity, cytoplasmic osmolarity, and Na+/H+ antiport activity as well as their resistance to ionic and osmotic stress. The growth studies show that ZH24K is more resistant to osmotic stress while ZHKPP is more resistant to salt stress. The studies on the stress-induced inactivation of photosynthetic machineries of mutants and wild type indicated that, for the photosystem I, ZH24K is the most resistant to osmotic induced inactivation and ZHKPP is the most resistant to ionic induced inactivation. For the photosystem II, both ZH24K and ZHKPP show increased resistance to either ionic or osmotic induced inactivation compared to that of wild type; however, ZH24K is more sensitive to ionic induced inactivation than ZHKPP and vice versa in case of osmotic induced inactivation. Comparing the lipid profile of the mutants and the wild type also suggests that the pgt gene product is functionally related to the synthesis of a group of polar glycolipids involving in stress responses.