INSTABILITY OF PLASMID-ENCODED CITRATE PERMEASE IN LEUCONOSTOC

The conversion from citrate positive (Cit(+)) to citrate negative (Cit (-)) phenotype of six strains of Leuconostoc mesenteroides was followe d during growth in milk and buffered or unbuffered MRS medium at 30 or 37 degrees C. High rate of loss of Cit(+) phenotype was observed. The Cit(-) phenotype was found to be linked to the loss of 22 to 23 kb pl asmids. All Cit(-) mutants isolated from Leuc. mesenteroides subsp. cr emoris 195 reverted spontaneously to the Cit(+) phenotype. Hybridizati on experiments using a 0.8 kb fragment of the citP gene of Leuc. mesen teroides showed that all the plasmids which were lost in Cit(-) mutant s encoded for a citrate permease. However, neither plasmid nor genomic DNA from Leuc. mesenteroides subsp. cremoris 195 hybridized with the citP probe.