Alpha-
galactosides are abundant
sugars in legumes such as soy. Because of the lack of
alpha-galactosidase (alpha-Gal) in the digestive tract, humans are unable to digest these
sugars, which consequently induce
flatulence. To develop the consumption of the otherwise highly nutritional soy products, the use of exogenous alpha-Gal is promising. In this framework, we characterized the melA gene for alpha-Gal in Lactobacillus plantarum. The melA gene encodes a cytoplasmic 84-kDa
protein whose enzymatically active form occurs as oligomers. The melA gene was cloned and expressed in Escherichia coli, yielding an active alpha-Gal. We show that melA is transcribed from its own promoter, yielding a monocistronic
mRNA, and that it is regulated at the transcriptional level, i.e., it is induced by
melibiose but is not totally repressed by
glucose. Posttranscriptional regulation by the
carbon source could also occur. Upstream of melA, a putative galactoside transporter, designated RafP, was identified that shows high homology to LacS, the unique transporter for both alpha- and beta-
galactosides in Streptococcus thermophilus. rafP is also expressed as a monocistronic
mRNA. Downstream of melA, the lacL and lacM genes were identified that encode a heterodimeric
beta-galactosidase. A putative galM gene identified in the same cluster suggests the presence of a
galactose operon. These results indicate that the genes involved in galactoside catabolism are clustered in L. plantarum ATCC 8014. This first genetic characterization of melA and of its putative associated transporter, rafP, in a lactobacillus opens doors to various applications both in the manufacture of soy-derived products and in probiotic and nutraceutical issues.