Replication origin mutations affecting binding of pSC101 plasmid-encoded Rep initiator protein

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• The pSC101 plasmid is a low-copy-number bacterial plasmid crucial for molecular cloning.
• Replication initiation of pSC101 depends on the Rep initiation protein binding to specific sites within the origin of replication.

Purpose of the Study:

• To elucidate the functional significance of individual Rep protein binding sites within the pSC101 minimal origin of replication.
• To determine the impact of mutations in these binding sites on plasmid replication, copy number, and stability.

Main Methods:

• Construction of pSC101 plasmids with various mutations in the three Rep binding sites (RS1, RS2, RS3).
• Assessing Rep protein affinity for the origin using competition binding assays.
• Measuring transformation efficiency, plasmid copy number, and stability over 150 generations for mutant plasmids.

Main Results:

• Mutating all three Rep binding sites reduced Rep protein affinity by 100-fold.
• Individual site mutations differentially affected Rep binding and plasmid replication.
• Mutation in RS1 (proximal to DnaB helicase entry sites) severely impaired replication.
• Mutation in RS3 (distal site) reduced copy number but maintained plasmid stability.
• Mutation in RS2 (middle site) had no significant effect on pSC101 replication.

Conclusions:

• The Rep binding sites within the pSC101 origin are not functionally equivalent.
• The RS1 binding site plays a critical role in pSC101 replication initiation and efficiency.
• Plasmid stability can be maintained even with reduced copy number and broader copy number distribution, depending on the specific mutation.