[Fragmentation of the lactose-protease plasmid in lactose-negative derivatives of Streptococcus lactis and S. lactis ssp. diacetylactis].

DNA analysis showed the presence of plasmids in all 29 strains of mesophilic lactic streptococci. Their number ranged from 1 to 9 and their molecular weight extended from 1.8 to 45 megadaltons (Md). Comparison of the plasmid profiles from 9 strains of Streptococcus lactis, 3 strains of S. lactis ssp. diacetylactis and from their Lac-,Prt-variants was carried out. In most of the strains, a single plasmid of 27 to 45 Md carried both the lactose and protease (lac and prt) determinants. In Lac- variants, this composite plasmid may at times be lost; in most derivatives, this plasmid was fragmented into several smaller plasmids. Fragmentation resulted in the inactivation of lac genes and often of prt genes; sometimes prt genes remained intact in one of these new plasmids. Fragmentation of the plasmid may also take place without inactivation of these metabolic genes. Restriction analysis of the lactose-protease plasmid from 3 strains of S. lactis and from one Lac-variant confirmed the plasmid fragmentation phenomenon, and led to a proposal for physical mapping of the plasmid and preliminary localization of the lac-prt genes.