Effect of Porphyridium sp. polysaccharide on malignant cell transformationby moloney murine sarcoma virus

The polysaccharide produced by the red microalga Porphyridium sp. was highl y inhibitory for cell transformation of mouse fibroblast cells by an MSV-12 4 virus stock. This inhibition was most effective if the polysaccharide was added 2 h before or at the time of infection. The finding that the inhibit ion of cell transformation by MuSV-124 was reversible after removal of the polysaccharide suggested that Porphyridium sp. polysaccharide inhibited a l ate step after provirus integration into the host genome. Addition of the p olysaccharide post-infection significantly reduced the number of transforme d cells, but its effect was less marked than that obtained when the polysac charide was added before or at the time of infection. These findings suppor t the possibility that the polysaccharide may affect early steps in virus r eplication cycle, such as virus absorption into the host cells, in addition to its effect on a late step after provirus integration.