Characterization and molecular cloning of cryptic plasmids isolated from Lactobacillus casei.

Four small cryptic plasmids were isolated from Lactobacillus casei strains, and restriction endonuclease maps of these plasmids were constructed. Three of the small plasmids (pLZ18C, pLZ19E, and pLZ19F1; 6.4, 4.9, and 4.8 kilobase pairs, respectively) were cloned into Escherichia coli K-12 by using pBR322, pACYC184, and pUC8 as vectors. Two of the plasmids, pLZ18C and pLZ19E, were also cloned into Streptococcus sanguis by using pVA1 as the vector. Hybridization by using nick-translated cloned 32P-labeled L. casei plasmid DNA as the probe revealed that none of the cryptic plasmids had appreciable DNA-DNA homology with the large lactose plasmids found in the L. casei strains, with chromosomal DNAs isolated from these strains. Partial homology was detected among several plasmids isolated from different strains, but not among cryptic plasmids isolated from the same strain.