Mutations in three loci in which affect fucosylation are described. Mutations in two of these loci resulted in the simultaneous loss of two separate carbohydrate epitopes. The GA-X epitope, which was competed by L-fucose, was absent in strains carrying a or mutation. These strains exposed a new carbohydrate epitope, competed by -acetylglucosamine, and the size of several glycoproteins was reduced. A second epitope (GA-XII) was also absent in strains carrying the or mutations, reducing the size of the glycoprotein which normally expresses it. Fucose content was reduced in the three mutants, suggesting that each mutation affected a separate step in fucosylation. The lesions did not appear to inhibit synthesis of the underlying carbohydrate, because detergent extracts of mutant vesicles were more active than normal vesicles at transferring [C]fucose from GDP-[C]fucose to endogenous acceptor species. The and mutant strains incorporated exogenous [H]fucose poorly, suggesting that lesions in the and genes interfere with the biosynthesis of fucoconjugates downstream from the previously described GDP-fucose synthesis defect of the mutation. Intac mutant vesicles were relatively inefficient in assays, suggesting a global fucosylation defect (which is consistent with the loss of both glycoantigens, GA-X and GA-XII, in this mutant). Finally, the mutation led to delayed accumulation of slime sheath The three genetic loci define a fucosylation pathway in comprising defined biochemical steps which contribute to multicellular morphogenesis in this organism.


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