Abstract
Alginate is an industrially widely used polysaccharide produced by
brown seaweeds and as an exopolysaccharide by bacteria belonging
to the genera Pseudomonas and Azotobacter. The polymer is composed
of the two sugar monomers mannuronic acid and guluronic acid (G),
and in all these bacteria the genes encoding 12 of the proteins essential
for synthesis of the polymer are clustered in the genome. Interestingly,
1 of the 12 proteins is an alginate lyase (AlgL), which is able to
degrade the polymer down to short oligouronides. The reason why this
lyase is associated with the biosynthetic complex is not clear, but
in this paper we show that the complete lack of AlgL activity in
Pseudomonas fluorescens in the presence of high levels of alginate
synthesis is toxic to the cells. This toxicity increased with the
level of alginate synthesis. Furthermore, alginate synthesis became
reduced in the absence of AlgL, and the polymers contained much less
G residues than in the wild-type polymer. To explain these results
and other data previously reported in the literature, we propose
that the main biological function of AlgL is to degrade alginates
that fail to become exported out of the cell and thereby become stranded
in the periplasmic space. At high levels of alginate synthesis in
the absence of AlgL, such stranded polymers may accumulate in the
periplasm to such an extent that the integrity of the cell is lost,
leading to the observed toxic effects.
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