Table 1.
Comparison of the different modes of plasmid replicationa
| Characteristic | Theta A | Theta Bb | Theta C | Theta D | Strand displacement | Rolling-circle |
|---|---|---|---|---|---|---|
| Directionality of replication (unidirectional vs bi-directional) | Bi/uni | Uni | Uni | Uni | Bi | Uni |
| Coupling of leading- and lagging-strand synthesis | Coupled (replisome) | Coupled (replisome) | Coupled (replisome) | Coupled (replisome) | Uncoupled | Uncoupled |
| Simultaneous leading- and lagging-strand synthesis | Yes | Yes | Yes | Yes | Yes | No, with generation of single-stranded intermediate |
| Leading strand synthesis | ||||||
| Continuous vs discontinuous | Continuous | Continuous | Continuous | Continuous | Continuous | Continuous |
| Initiation factor | Rep melts duplex and recruits DnaB helicase, often assisted by DnaA and by NAPs | NA | RepA (also primase activity) | Rep E melts duplex only and has active role in primer generation | Rep C (DnaA-like) | Rep (generates initial nick and seals it) |
| Initiation factor binding site | Multiple iterons with induced bending | NA | Iteron with two functional subregions: (I) binding and (II) initiation of DNA replication | No iterons, no bending | ssi sites (hairpins) | Dso (hairpin cruciform structure with binding domain and nicking domain) |
| Leading-strand primer | DnaG primase (host) | Transcript processed by RNaseH1 | ppApGpA produced by RepA | RepE transcript, possibly processed by RepE itself | RepB′ (interacting at the base of the hairpin) | Rep C-generated nick produces a 3′-OH |
| Helicase | DnaBb | PriA + DnaBb | PriA + DnaBb | PriA + DnaBb | RepA | DnaBb |
| Lagging-strand synthesis | ||||||
| Continuous vs discontinuous | Discontinuous | Discontinuous | Discontinuous | Discontinuous | Continuous | Continuous |
| Lagging-strand primer | DnaG | DnaG | DnaG | DnaG | RepB′ | DnaG |
| Helicase | PriA+ DnaBb | PriA+ DnaBb | PriA+ DnaBb | PriA+ DnaBb | RepA in coordination with Pol III | Not needed (it is ssDNA) |
| Additional comments | Predominant type in Gram-negative bacteria | Limits size of the plasmid to ∼40kb | Minimal size of cis-acting ori | 2 sites of replisome arrest: for Top I-like enzyme and for resolvase | More independent of host factorsLimits the size of the plasmid to ∼15 kb | Predominant type in Gram-positive bacteria, also associated with conjugation |