Аннотация
Alginate is a linear copolymer of beta-d-mannuronic acid and its C-5-epimer,
alpha-l-guluronic acid. During biosynthesis, the polymer is first
made as mannuronan, and various fractions of the monomers are then
epimerized to guluronic acid by mannuronan C-5-epimerases. The Azotobacter
vinelandii genome encodes a family of seven extracellular such epimerases
(AlgE1 to AlgE7) which display motifs characteristic for proteins
secreted via a type I pathway. Putative ATPase-binding cassette regions
from the genome draft sequence of the A. vinelandii OP strain and
experimentally verified type I transporters from other species were
compared. This analysis led to the identification of one putative
A. vinelandii type I system (eexDEF). The corresponding genes were
individually disrupted in A. vinelandii strain E, and Western blot
analysis using polyclonal antibodies against all AlgE epimerases
showed that these proteins were present in wild-type culture supernatants
but absent from the eex mutant supernatants. Consistent with this,
the wild-type strain and the eex mutants produced alginate with about
20% guluronic acid and almost pure mannuronan (< or =2% guluronic
acid), respectively. The A. vinelandii wild type is able to enter
a particular desiccation-tolerant resting stage designated cyst.
At this stage, the cells are surrounded by a rigid coat in which
alginate is a major constituent. Such a coat was formed by wild-type
cells in a particular growth medium but was missing in the eex mutants.
These mutants were also found to be unable to survive desiccation.
The reason for this is probably that continuous stretches of guluronic
acid residues are needed for alginate gel formation to take place.
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