title: Evidence for a Plasmid-Linked Restriction-Modification System in Lactobacillus helveticus.
abstract: The presence of a restriction-modification (R/M) system against two bacteriophages, 328-B1 and hv, was demonstrated in three Lactobacillus helveticus strains, CNRZ 1094, CNRZ 1095, and CNRZ 1096. In addition, the burst size of phage 328-B1 in the three restrictive strains CNRZ 1094, CNRZ 1095, and CNRZ 1096 was reduced with respect to the values obtained in its propagating strain, CNRZ 328. Heating at 60 degrees C did not inactivate the R/M system. Nonrestrictive variants from CNRZ 1094 were easily obtained under several culture conditions, but treatment with novobiocin at 42 degrees C followed by storage at -20 degrees C resulted in drastic elimination of the R/M phenotype from all clones tested. Electrophoretic analysis of CNRZ 1094 nonrestrictive variants revealed the concomitant loss of a 34-kb plasmid. Four EcoRI fragments from the 34-kb plasmid were cloned in the Escherichia coli vector pACYC184. The use of one or several of these fragments as probes confirmed the plasmidic location of the genes responsible for the R/M system. These probes also showed the presence of R/M plasmids in the two other restrictive strains, CNRZ 1095 and CNRZ 1096. Lactose-fermenting ability and/or proteolytic capacity was not linked to the 34-kb plasmid.
authors.affiliation: Station de Recherches Laitières, Institut National de la Recherche Agronomique, 78350 Jouy-en-Josas, France.
authors.forename: C G
authors.initials: CG
authors.lastname: de Los Reyes-Gavilán
authors.forename: G K
authors.initials: GK
authors.lastname: Limsowtin
authors.forename: L
authors.initials: L
authors.lastname: Séchaud
authors.forename: M
authors.initials: M
authors.lastname: Veaux
authors.forename: J P
authors.initials: JP
authors.lastname: Accolas