~ ~ ~~The aga gene coding for a-galactosidase in Streptococcus mutans was detected in a recombinant gene library constructed in phage A. The gene was subcloned into plasmid vectors and shown to specify a novel protein of M, 8OOOO. Characterization of a-galactosidase from S. mutans and from recombinant Escherichia coli expressing aga indicated that the enzyme functions as a tetramer. The amino acid composition of the a-galactosidase, deduced from nucleotide sequencing of aga, gave a predicted M, of 82022 and revealed regions of homology to agalactosidases encoded by the E. coli Raf plasmids and by Bacillus stearothermophilus. Inactivation of the aga gene in S. mutans resulted in loss of all a-galactosidase activity and abolished the ability to ferment melibiose; aglucosidase activity was also lost, due to an indirect effect on the dexB gene.
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