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Applied and Environmental Microbiology, September 1999, p. 3955-3963, Vol. 65, No. 9
Institut für Industrielle Genetik,
Universität Stuttgart, 70569 Stuttgart, Germany
Received 25 March 1999/Accepted 29 June 1999
An
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Cloning of the Gene Encoding a Novel
Thermostable
-Galactosidase from Thermus
brockianus ITI360
-galactosidase gene from Thermus brockianus
ITI360 was cloned, sequenced, and expressed in Escherichia
coli, and the recombinant protein was purified. The gene,
designated agaT, codes for a 476-residue polypeptide with a
calculated molecular mass of 53,810 Da. The native structure of the
recombinant enzyme (AgaT) was estimated to be a tetramer. AgaT displays
amino acid sequence similarity to the
-galactosidases of
Thermotoga neapolitana and Thermotoga maritima
and a low-level sequence similarity to
-galactosidases of family 36 in the classification of glycosyl hydrolases. The enzyme is
thermostable, with a temperature optimum of activity at 93°C with
para-nitrophenyl-
-galactopyranoside as a substrate. Half-lives of inactivation at 92 and 80°C are 100 min and 17 h, respectively. The pH optimum is between 5.5 and 6.5. The enzyme displayed high affinity for oligomeric substrates. The
Kms for melibiose and raffinose at 80°C were
determined as 4.1 and 11.0 mM, respectively. The
-galactosidase gene
in T. brockianus ITI360 was inactivated by integrational
mutagenesis. Consequently, no
-galactosidase activity was detectable
in crude extracts of the mutant strain, and it was unable to use
melibiose or raffinose as a single carbohydrate source.
*
Corresponding author. Mailing address: Institut
für Industrial Genetik, Universität Stuttgart, 70569 Stuttgart, Germany. Phone: 49 (0) 711 685 6982. Fax: 49 (0) 711 685 6973. E-mail: olaf{at}genius.biologie.uni-stuttgart.de.
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