ANALYSIS OF THE PHYSICAL AND FUNCTIONAL-CHARACTERISTICS OF CELL CLUMPING IN LACTOSE-POSITIVE TRANSCONJUGANTS OF LACTOCOCCUS-LACTIS SSP LACTIS ML3

Transconjugants of Lactococcus lactis ssp. lactis ML3 that acquire the ability to utilize lactose often exhibit self-aggregation and become able to transfer the ability to utilize lactose at frequencies 10(2) t o 10(5) times higher than strain ML3. Our laboratory investigated the physical and functional characteristics of cell aggregation in ML3 tra nsconjugants. Results showed that donor cell autoaggregation was disso ciated when cells were suspended in buffers that contained EDTA or whe n they were briefly exposed to proteinase K or alpha-chymotrypsin. Con jugation studies revealed that disruption of donor cell aggregation su bstantially decreased the efficiency of lactose plasmid transfer. Diss ociation by EDTA or proteolytic enzyme treatments was reversible, and recovery of high frequency lactose plasmid transfer ability accompanie d the restoration of donor self-aggregation. Analysis of cell-surface proteins isolated from lactose-positive, aggregation-positive and from lactose-positive, aggregation-negative transconjugants of ML3 by PAGE indicated that aggregation-positive cells produced a unique protein o f approximately 125 kDa. The results suggested that cell aggregation w as essential for high frequency lactose plasmid transfer in ML3 transc onjugants and that at least one large protein was involved in aggregat ion.