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Characterization of a theta-replicating plasmid from Streptococcus thermophilus
Nathalie Turgeon1, Michel Frenette, Sylvain Moineau
1Département 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
|January 9, 2004
Summary
This study characterizes Streptococcus thermophilus plasmids, revealing novel replication mechanisms. Group D plasmids use rolling-circle replication, while group B plasmids exhibit theta replication, a first for this species.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Streptococcus thermophilus plasmids were previously classified into four groups (A-D) based on DNA homology.
- Characterization of group B and D plasmids provides new insights into their genetic makeup and replication strategies.
Purpose of the Study:
- To characterize the plasmids of Streptococcus thermophilus belonging to groups B and D.
- To investigate the replication mechanisms and genetic elements of these plasmids.
- To propose a new classification of sequenced S. thermophilus plasmids based on replication modes.
Main Methods:
- Sequence analysis of pSMQ173b (group D) and pSMQ308 (group B).
- Identification of open reading frames (ORFs) and prediction of their functions.
- Northern blot experiments to confirm gene transcription.
- Two-dimensional agarose gel electrophoresis to determine plasmid replication mechanisms.
Main Results:
- pSMQ173b (group D) is 4449 bp with five ORFs, replicating via the rolling-circle mechanism.
- pSMQ308 (group B) is 8144 bp with six ORFs, including genes for primase/helicase and integrase.
- Transcription of these two ORFs was confirmed in group B strains.
- pSMQ308 replicates via the theta mechanism, the first reported instance in S. thermophilus.
- A proposed classification of 20 sequenced S. thermophilus plasmids into six groups based on replication mode.
Conclusions:
- The characterization of group B and D plasmids expands our understanding of Streptococcus thermophilus plasmid diversity.
- The identification of theta replication in a group B plasmid represents a novel finding for this species.
- A new classification system for S. thermophilus plasmids based on replication mechanisms is proposed, aiding future research.