Molecular description and industrial potential of Tn6098 conjugative transfer conferring alpha-galactoside metabolism in Lactococcus lactis

A novel 51-kb conjugative transposon of Lactococcus lactis, designated Tn6098, encoding the capacity to utilize alpha-galactosides such as raffinose and stachyose, was identified and characterized. Alpha-galactosides are a dominant carbon source in many plant-derived foods. Most dairy lactococcus strains are unable to use alpha-galactosides as a growth substrate, yet many of these strains are known to have beneficial industrial traits. Conjugal transfer of Tn6098 was demonstrated from the plant-derived donor strain L. lactis KF147 to the recipient L. lactis NZ4501, a derivative of the dairy model strain L. lactis MG1363. The integration of Tn6098 into the genome of the recipient strain was confirmed by Illumina sequencing of the transconjugant L. lactis NIZO3921. The molecular structure of the integration site was confirmed by a PCR product spanning the insertion site. A 15-bp direct repeat sequence (TTATACCATAATTAC) is present on either side of Tn6098 in the chromosome of L. lactis KF147. One copy of this sequence is also present in the L. lactis MG1363 chromosome and represents the sole integration site. Phenotypic characterization of all strains showed that the transconjugant has not only acquired the ability to grow well in soy milk, a substrate rich in alpha-galactosides, but also has retained the flavor-forming capabilities of the recipient strain L. lactis MG1363. This study demonstrates how (induced) conjugation can be used to exploit the beneficial industrial traits of industrial dairy lactic acid bacteria in fermentation of plant-derived substrates