Abstract
An Azotobacter vinelandii mannuronan C-5-epimerase gene was cloned
in Escherichia coli. This enzyme catalyzes the Ca(2+)-dependent epimerization
of D-mannuronic acid residues in alginate to the corresponding epimer
L-guluronic acid. The epimerase gene was identified by screening
a bacteriophage EMBL3 gene library of A. vinelandii DNA with a synthetic
oligonucleotide probe. The sequence of this probe was deduced after
determination of the N-terminal amino acid sequence of a previously
reported extracellular mannuronan C-5-epimerase from A. vinelandii.
A DNA fragment hybridizing against the probe was subcloned in a plasmid
vector in E. coli, and the corresponding recombinant plasmid expressed
intracellular mannuronan C-5-epimerase in this host. The nucleotide
sequence of the gene encoding the epimerase was determined, and the
sequence data showed that the molecular mass of the deduced protein
is 103 kDa. A module consisting of about 150 amino acids was repeated
tandemly four times in the C-terminal part of the deduced protein.
Each of the four repeats contained four to six tandemly oriented
nonameric repeats. The sequences in these motifs are similar to the
Ca(2+)-binding domains of functionally unrelated secreted proteins
reported previously in other bacteria. The reaction product of the
recombinant epimerase was analyzed by nuclear magnetic resonance
spectroscopy, and the results showed that the guluronic acid residues
were distributed in blocks along the polysaccharide chain. Such a
nonrandom distribution pattern, which is important for the commercial
use of alginate, has previously also been identified in the reaction
product of the corresponding enzyme isolated from A. vinelandii.
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