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Characterization of a theta-replicating plasmid from Streptococcus thermophilus.

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.
Plasmid (Impact Factor: 1.76). 02/2004; 51(1):24-36. DOI: 10.1016/j.plasmid.2003.09.004
Source: PubMed

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.

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