CH391L/S12/Origins of Replication
Origins of Replication, overview
In order for a piece of circular, dsDNA to be propagated in bacteria, it needs to be replicated by host machinery. There is a sequence in the plasmid that directs the cell to begin replication. Important considerations are host range, compatibly, and copy number. The host range refers to what species of bacteria will recognize the origin of replication and thus allow for replication. The compatibility refers to a plasmid's ability to coexist with another plasmid in the same cell. Copy number refers to the average or expected number of copies of the plasmid per cell.
There are three main mechanisms for plasmid replication: Rolling Circle, Strand Displacement, and Theta.
- Strand Displacement...
- In Rolling Circle Replication, a nick is made by the Rep protein at the "double strand origin" of a dsDNA plasmdid. The free 3'OH is extended, displacing as it progresses. After one unit length of displacement, replication is terminated, yielding one dsDNA plasmid and ssDNA of one unit length. The displaced strand then serves as a template for replication from a "single strand origins." Since each strand is replicated independently, it is possible for the ssDNA form to accumulate. This mechanism is found in Staphylococcus aureus and Streptomyces lividans as well as many bacteriophages.
- Theta replication is the most common form of DNA replication, including most plasmids as well as chromosomes. RNA serves a primer, DNA is polymerized in one or both directions. In the first case, a single fork circumnavigates the entire plasmid until the origin is reached, and daughter plasmids separate. In bidirectional replication, two forks propagate and meet on the far side of the plasmid before resolution. ColE1, p15A, RK2, F, and P1 all use theta replication.
Plasmids are classified as having a narrow or broad host range.
- ColE1 and pMB1 are limited to E. coli and a few close relatives,
- RK2 plasmids can be used in most gram-negative bacteria.
- RSF1010 can use used in most gram-negative bacteria, and some gram-positive
- Plasmids from gram-positive bacteria tend to function well in other gram-positive bacteria.
If two plasmids have the same (or very similar) origins of replication, they will compete with each other for replication machinery. This results in an unstable situation. If the two plasmids posses different selectable markers, this can be maintained for several generations, but eventually one of the plasmids will be lost. For scenarios in which multiple plasmids are necesary, one must be careful to choose plasmids will compatible origins. The most common dual-plasmid pair is ColE1(or pMB1) and p15A. The most common plasmid triplet is ColE1 (or pMB1),p15A, and pSC101.
- ColE1 and pMB1 are closely related and fall into the same compatibility group
- ColE1: pBlue
- pMB1: pUC, pBR322, pGEM, pET,
- p15A: pBad, pACYC
- ColE1: 15-20 copies
- pMB1: 20-700 copies
- pUC: 500-700 copies
- pBR322: ~20 copies
- pSC101: ~5 copies
- p15A: 10-12 copies
- F1: ~1 copy
- ColDF13: 20-40 copies
- ColA: 20-40 copies
- RSF1030: >100 copies
- P1:~1 copy
Control of Initiation/copy number
- RNA regulation:
- ColE1/pMB1: Does not require plasmid encoded initiator protein, does require DnaA binding. The origin contains regions promoting the synthesis of RNA I and RNA II. RNA II hybrizes to the DNA, yielded a DNA/RNA hybrid which can serve as a substrate for RNaseH. Digestion of RNA II by RNaseH yields the primer for replication. RNA I binds and sequesters RNA II, so it is unavailable for RNAse H digestion. Since RNA II is "used" each time the plasmid replicates, and RNA I is not used, the ratio of RNA I to RNA II increases with the copy number. This provides a negative feedback for replication, and sets the average number of plasmids per cell.
The Rop protein helps lower the copy number, by stabilizing the RNA I/ RNA II duplex. Deletion of Rop, as well as a point mutation that weakens the RNA I and RNA II duplex, accounts for the higher copy of pUC (a pMB1 derivatives)
- p15A: similar to ColE1, but with different versions of RNA I and RNA II.
- RNA and protein regulation:
- R1: oriR is bound by RepA promoting initiation. No iterons. Copy number controlled by copA and copB. copA is in RNA that binds the 5’ end of RepA RNA and prevents translation. CopB blocks repA transcription.
- Iteron regulation
- pSC101, F, RK6, P1, RK2, RP4
- del Solar G, Giraldo R, Ruiz-Echevarría MJ, Espinosa M, and Díaz-Orejas R. . pmid:9618448.
- Lin-Chao S, Chen WT, and Wong TT. . pmid:1283002.
Regarding the copy number of pUC vs pBR322
- and Genetic Elements)