highcopy

Highcopy refers to high-copy plasmids in molecular biology, a class of plasmid vectors that exist in many copies per host cell and thereby increase gene dosage. Copy number is the average number of plasmid molecules per cell and influences the level of expression of cloned inserts. High-copy plasmids typically achieve tens to hundreds of copies per cell, with ColE1-type origins commonly yielding several hundred copies under standard growth conditions. In contrast, low-copy plasmids maintain a much smaller number of copies per cell.

Origins of replication govern copy number through regulatory RNA and initiator proteins. ColE1-type plasmids use an antisense RNA (RNA I) and a primer RNA (RNA II) to regulate initiation of replication; mutations in the origin or regulatory RNAs can raise copy number, while increased plasmid burden can reduce growth rate and stability. Host conditions such as growth phase, temperature, and medium also modulate replication.

Applications include high-level expression of cloned genes, screening, and DNA library construction. The high gene dosage from these plasmids can improve detectable signals in expression systems or in selection-based screening. However, the metabolic burden and plasmid instability associated with high copy numbers can reduce cell viability and plasmid maintenance, particularly with large inserts or toxic genes. To balance expression and stability, researchers may use inducible promoters, lower-copy variants, or plasmids with partitioning systems.

Examples: pUC19 and related ColE1-derived vectors are widely used high-copy plasmids in Escherichia coli. See also plasmid copy number, origin of replication, and plasmid stability.