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Molecular properties of Streptococcus thermophilus plasmid pER35 encoding a restriction modification system
1Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, USA.
Current Microbiology
|January 3, 2001
Summary
Bacteriophages threaten dairy production. This study sequenced plasmids from Streptococcus thermophilus, finding a novel restriction-modification system on pER35 that likely protects against phage attack.
Area of Science:
- Microbiology
- Molecular Biology
- Dairy Science
Background:
- Bacteriophage (phage) infections cause significant economic losses in the dairy industry by targeting lactic fermentation bacteria (LFB).
- LFB possess defense mechanisms, including restriction of foreign DNA, to combat phage predation.
- Streptococcus thermophilus, a key thermotolerant LFB, can be susceptible to phage attacks, impacting dairy fermentation processes.
Purpose of the Study:
- To investigate plasmid-borne genetic elements in Streptococcus thermophilus strains relevant to dairy fermentation.
- To identify potential mechanisms of phage resistance in S. thermophilus.
- To explore the genetic basis of phage defense in thermotolerant LFB.
Main Methods:
- Whole-genome sequencing of three plasmids (pER16, pER35, pER36) from Streptococcus thermophilus strains.
- Bioinformatic analysis to identify encoded genes and compare them with known systems.
- Comparative analysis of restriction-modification (R-M) systems across different LFB species.
Main Results:
- Plasmid pER35 from S. thermophilus ST135 encodes a type IC restriction-modification (R-M) system highly similar to those found in Lactococcus lactis.
- The R-M system on pER35 is likely functional and provides phage resistance, analogous to systems in mesophilic L. lactis.
- Plasmid pER16 encodes only the specificity subunit of an R-M system; all three plasmids carry genes for replication (RepA) and stress response (Hsp).
Conclusions:
- The discovery of a complete R-M system on a plasmid in S. thermophilus is novel, as this species often lacks plasmids.
- This plasmid-encoded R-M system represents a significant mechanism for protecting S. thermophilus against bacteriophage predation during dairy fermentation.
- The high similarity of R-M systems suggests potential horizontal gene transfer between different LFB species, contributing to their collective phage resistance strategies.