Mutational and physiological analyses of plasmid pT181 functions expressing incompatibility

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Plasmid pT181, a multicopy plasmid from Staphylococcus aureus, belongs to incompatibility group 3.
• It exhibits two distinct incompatibility mechanisms: Inc3A and Inc3B.
• Inc3A involves the copA determinant and its transcripts (RNA I and RNA II) inhibiting RepC initiator protein synthesis.
• Inc3B is mediated by the leading strand replication origin and competition for RepC.

Purpose of the Study:

• To investigate the roles of copA and the replication origin in pT181 plasmid incompatibility.
• To analyze the effects of mutations within these regions on plasmid replication and stability.
• To understand the mechanisms underlying Inc3A and Inc3B incompatibility in pT181.

Main Methods:

• Cloning of the copA region from 11 pT181 copy number mutants onto a compatible vector (pE194).
• Testing the inhibitory effects of cloned copA regions on pT181 replication.
• Cloning and testing the pT181 replication origin for its inhibitory effects.
• Assessing the resistance of copy mutants to Inc3B inhibition.

Main Results:

• Mutations in copA significantly reduced or eliminated Inc3A activity.
• All copy mutants displayed resistance to Inc3B inhibition, unlike the wild-type.
• Cloned wild-type copA and origin regions reduced pT181 copy number proportionally to gene dosage but did not completely block replication.
• The cloned incompatibility determinants interfered with the plasmid's copy correction mechanism, destabilizing the plasmid.

Conclusions:

• The study elucidates the distinct roles of copA and the replication origin in pT181 plasmid incompatibility.
• Mutations affecting RNA I and RNA II production or sequence disrupt Inc3A.
• The findings suggest that incompatibility determinants destabilize plasmids by interfering with copy correction mechanisms.