Assimilation of sulphur into the cell-wall polysaccharide of the red microalga Porphyridium sp (Rhodophyta)

The cell walls of red algae contain sulphated polysaccharides. It is believed that the bioactivity of such natural sulphated polysaccharides may be attributed to the sulphate groups. Knowledge of the mode of sulphation of these polysaccharides is, however, very limited. The current study forms part of our efforts to elucidate the mode of sulphation of the cell-wall polysaccharide of the red microalga Porphyridium sp. Although this microalga is an obligatory photoautotroph, under conditions of sulphate starvation it can assimilate the sulphur-containing amino acid cysteine (but not methionine) and incorporate sulphur from [S-35]cysteine into the cell-wall polysaccharide. The ratio between S-35 uptake to the soluble polysaccharide complex and to the cells was threefold higher from [S-35]cysteine than that from (Na2SO4)-S-35, although the uptake of S-35 to the cells was higher when (Na2SO4)-S-35 was used as the sulphur source. Pulse-chase experiments demonstrated that differences in the incorporation of 35S from the two sulphur sources were greater for the soluble polysaccharide complex (45.1% from [S-35]cysteine vs. 13.9% from (Na2SO4)-S-35) than for the cellular fractions (bound polysaccharide, protein and low-molecular-weight fraction). SDS-PAGE analysis showed that the S-35 label resided predominantly in the polysaccharide of the soluble complex and not in the glycoprotein. The effect of sodium chlorate, a sulphation inhibitor, on S-35 incorporation into the soluble polysaccharide complex and the different cellular fractions depended on the S-35 source: significant inhibition of sulphation of the soluble polysaccharide complex was found for [S-35]cysteine as the source of sulphate, i.e. 87% vs. 36.4% for (Na2SO4)-S-35. This study suggests that in addition to the commonly accepted sulphate pathway, there is another sulphation pathway in which cysteine serves as the sulphur source.