Cloning of chromosomal genes of Lactococcus by heterologous complementation: partial characterisation of a putative lactose transport gene.

A cosmid gene library of the genome of Lactococcus lactis subsp. lactis 712 has been constructed in the broad host range plasmid pLAFR1 in Escherichia coli LE392. Three lactococcal genes from the bank were identified by heterologous complementation of specific mutations in strains of E. coli. A cosmid clone encoding a putative lactose transport gene was identified by complementing an E. coli lacY mutant. The complemented clone supported the uptake of 14C lactose in transport assays. The DNA fragment responsible was subcloned and localised to a 1.28 kb fragment of the lactococcal chromosome.