INSTABILITY OF PLASMID ENCODING FOR BETA-GALACTOSIDASE IN LEUCONOSTOC-MESENTEROIDES SUBSP MESENTEROIDES

The appearance of Lac(-) variants of Leuconostoc mesenteroides subsp. mesenteroides 19D was studied in different growth conditions, at 30 an d 37 degrees C, in milk and buffered or unbuffered MRS medium. Growth in milk resulted in greater stability of the lactose phenotype. The hi ghest frequency of Lac- variants (40%) appeared in the culture in whic h mortality was the highest (unbuffered MRS, 37 degrees C, 96 h). The growth kinetics and fermentation balances of Lac(-) variants and of th e parental strain were identical. No phospho-beta-galactosidase activi ty could be detected. The activity of beta-galactosidase was high afte r growth in lactose-containing medium and was low in glucose medium. G lucose totally repressed galactokinase activity. Enzymatic analyses of Lac(-) variants showed that the Lac(-) character was associated with the total absence of beta-galactosidase activity, in turn correlated w ith the loss of the 47.3 kb plasmid. An oligonucleotide probe correspo nding to a homologous sequence of Leuconostoc lactis beta-galactosidas e enabled the beta-galactosidase gene to be localized on this plasmid.