Transfer of Tn916 between Lactococcus lactis subsp. lactis strains is nontranspositional: evidence for a chromosomal fertility function in strain MG1363

Lactococcus lactis subsp. lactis MG1363 can act as a conjugative donor of chromosomal markers. This requires a chromosomally located fertility function that we designate the lactococcal fertility factor (Laff). Using inter- and intrastrain crosses, we identified other L. lactis strains (LMO230 and MMS373) that appear to lack Laff. The selectable marker in our crosses was Tcr, carried by Tn916, a transposon present on the chromosome. The transfer of Tcr was not due to Tn916-encoded conjugative functions, because (i) L. lactis cannot act as a donor in Tn916-promoted conjugation (F. Bringel, G. L. Van Alstine, and J. R. Scott, Mol. Microbiol. 5:2983-2993, 1992) and (ii) transfer occurred when the Tcr marker was present in a Tn916 derivative containing a mutation, tra-641, that prevents Tn916-directed conjugation in any host. In addition, we isolated a strain in which Tn916 appears to be linked to Laff; this strain should be useful for further analysis of this fertility factor. In this strain, Tn916 is on the same 600-kb SmaI fragment as Clu, a fertility factor previously shown to promote lactose plasmid transfer in L. lactis. Thus, it is possible that Clu and Laff are identical.