Cell-wall formation during the cell cycle of Porphyridium sp. (Rhodophyta)

The cell wall of the red microalgae Porphyridium sp, (UTEX 637) comprises a complex amorphous polysaccharide (6-7 x 10(6) Da). The polysaccharide is m ade up of xylose, glucose, and galactose as the main sugars, as well as som e minor sugars, protein, sulfate, and glucuronic acid, the latter two confe rring a negative charge on the polysaccharide. In this study, we used synch ronized cultures as one of the ways of unraveling the mechanism of biosynth esis of this complex polysaccharide by following cell-wall formation during the cell cycle. Synchronization of Porphyridium sp, was achieved with an a lternating light:dark regime of 12:12 h LD and dilution of the culture at t he end of the cycle. Under these conditions, cell duplication occurred betw een the 12th and 14th hours of the cycle. The following order of building t oward formation of the final polysaccharide appeared to take place: Interme diate polysaccharides with molecular masses ranging from 0.5 x 10(6) to 2 x 10(6) Da appeared in succession during hours 2-6 of the cycle, and the ful l-sized polysaccharide was detected by the 8th hour, At the beginning of th e cycle, xylose was the predominant sugar. Sulfur peaked at hours 2-4; gluc ose, galactose, and glucuronic acid at hours 8-12; and the minor sugars at hours 12-14, Upon incubation of low molecular mass polymer (0.5 x 10(6) Da) collected from the 4th hour with cellular crude extract from cells of the 6th hour of the cycle, two intermediates were formed (0.8 x 10(6) Da and 2 x 10(6) Da). We suggest that the 0.5 x 10(6) Da polymer intermediate, which is composed mainly of xylose, is the first polymer secreted into the mediu m, where it is further polymerized enzymatically to produce the 2 x 10(6) D a polymer via an intermediate 0.8 x 10(6) Da polymer. Later, the full-size polysaccharide is produced.