The lactose transporter in Leuconostoc lactis is a new member of the LacS subfamily of galactoside-pentose-hexuronide translocators.

The gene encoding the lactose transport protein (lacS) of Leuconostoc lactis NZ6009 has been cloned from its native lactose plasmid, pNZ63, by functional complementation of lactose permease-deficient Escherichia coli mutants. Nucleotide sequence analysis revealed an open reading frame with the capacity to encode a protein of 639 amino acids which had limited but significant identity to the lactose transport carriers (LacS) of Streptococcus thermophilus (34.5%) and Lactobacillus bulgaricus (35.6%). This similarity was present both in the amino-terminal hydrophobic carrier domain, which is homologous to the E. coli melibiose transporter, and in the carboxy-terminal enzyme IIA-like regulatory domain. The flanking regions of DNA surrounding lacS were also sequenced. Preceding the lacS gene was a small open reading frame in the same orientation encoding a deduced 95-amino-acid protein with a sequence similar to the amino-terminal portion of beta-galactosidase I from Bacillus stearothermophilus. The lacS gene was separated from the downstream beta-galactosidase genes (lacLM) by 2 kb of DNA containing an IS3-like insertion sequence, which is a novel arrangement for lac genes in comparison with that in other lactic acid bacteria. The lacS gene was cloned in an E. coli-Streptococcus shuttle vector and was expressed both in a lacS deletion derivative of S. thermophilus and in a pNZ63-cured strain, L. lactis NZ6091. The role of the LacS protein was confirmed by uptake assays in which substantial uptake of radiolabeled lactose or galactose was observed with L. lactis or S. thermophilus plasmids harboring an intact lacS gene. Furthermore, galactose uptake was observed in NZ6091, suggesting the presence of at least one more transport system for galactose in L. lactis.