title: Characterization of a theta-replicating plasmid from Streptococcus thermophilus.
abstract: Plasmids of Streptococcus thermophilus were previously classified, based on DNA homology, into at least four groups (A-D). Here, we report the characterization of plasmids of group B and D. The sequence analysis of pSMQ173b (group D) indicates that this plasmid contains 4449 bp, five open reading frames (ORFs) and replicates via the rolling-circle mechanism of the pGI3 family. The plasmid pSMQ308 (group B) contains 8144 bp and six ORFs. Two ORFs likely encode a primase/helicase and an integrase. Northern blot experiments demonstrate that these two orfs are transcribed within the three strains containing plasmids of group B. Two-dimensional agarose gel electrophoresis shows that pSMQ308 replicates via a theta mechanism. To our knowledge, this is the first report of a plasmid replicating via a theta mode in S. thermophilus. Finally, a classification of 20 sequenced S. thermophilus plasmids into six groups based on their mode of replication is proposed.
authors.affiliation: Département de biochimie et de microbiologie, Faculté des sciences et de génie, Groupe de recherche en écologie buccale, Faculté de médecine dentaire, Université Laval, Québec, Canada G1K 7P4.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Turgeon
authors.forename: Michel
authors.initials: M
authors.lastname: Frenette
authors.forename: Sylvain
authors.initials: S
authors.lastname: Moineau
mesh_terms: D000595:Amino Acid Sequence; D008969:Molecular Sequence Data; D010957:Plasmids / Q000145:classification; D017422:Sequence Analysis, DNA; D013291:Streptococcus / Q000235:genetics*