Plasmid copy number and control mechanism




Plasmid copy number and control mechanism
Plasmid copy number and control mechanism

Plasmid copy number:

  • The copy number plasmid means the number of plasmids that are normally present in a single bacterial cell.
  • The size and copy number of a plasmid is important.
  • Cloning is concerned with it.
  • Plasmid size that less than 10 kb is desirable for a cloning vector.
  • Some plasmids, especially the larger ones, are stringent.
  • Larger plasmids and have a low copy number which might be just one or two per cell.
  • Others are called relaxed plasmids which are present in multiple copies of 50 or more per cell.
  • Large quantities of the recombinant DNA molecule can be obtained when multiple copies of the cloning vector are present in the cell.
  • Naturally occurring plasmids are usually stably maintained in their bacterial hosts.
  • This stability often must be accomplished despite a very low number of plasmid copies per cell.

Plasmid replication and control:

  • Many plasmids are replicated as double-stranded circular molecules.
  • The replication of the double-stranded molecule is the same as that of the chromosome.
  • Replication starts from the origin of replication which is known as
  • It may be unidirectional ( single directional ) or bidirectional ( both directions).
  • When the whole circle is completed, then it segregates.
  • However, there are some aspects of replication that differ from that of the chromosome, especially for the multicopy plasmids.

Mechanism of regulation of plasmid copy number

  • Regulation of plasmid number is important to control the copy number of the plasmid.
  • The mechanisms used by some plasmids to regulate their copy number are :

1) ColE1-derived Plasmids:

  • It is the regulation of the processing of primer by complementary RNA
  • The mechanism of copy number regulation of the plasmid ColE1 was one of the first to be studied.
  • The mechanism of regulation of ColE1-derived plasmids is shown in:
    • Replication starts always from the origin of replication (oriV)
    • RNA II forms the primer for the plasmid DNA replication.
    • At the origin of replication, an RNA-DNA hybrid is formed.
    • RNA is RNA II and that DNA is the DNA of plasmid.
    • Then RNase cleaves the RNA II.
    • RNase H has endonuclease activity which means it can capacity to cleave.
    • At the cleaved or cut portion, 3 ʹOH is exposed where the nucleotides will be added by DNA polymerase I.
    • Thus the plasmid is replicated.
    • But when there is a sufficient number of plasmid its replication needs to be stopped.
    • This process is regulated by the RNA I and the Rop protein.
    • The Rop protein (sometimes called Rom)  helps RNA I to pair with RNA II.
    • RNA I is transcribed from the opposite strand to RNA II and is complementary to the first 108 bases of RNA II.
    • When there is pairing between RNA I and RNA II, RNAse can not cut the strand.
    • When the strand isn’t cleaved further replication is halted.
    • Therefore, it inhibits DNA replication.
    • The inhibition of replication is almost complete when the concentration of the plasmid reaches about 16 copies per cell which is the copy number of the original ColE1 plasmid.
    • Mutations that inactivate Rop cause only a moderate increase in the plasmid copy number.
    • Regulation of the replication of ColE1-derieved plasmid
      Fig. Regulation of the replication of ColE1-derieved plasmid
  1. R1 plasmid:
  • R1 plasmid is a member of the IncFII family of plasmids.
  • Plasmids belonging to the INFII group (R1, R6-5, R100) are self-transmissible, conjugative plasmids found in Enterobacteriaceae.
  • R1 plasmid regulates its copy number by regulating the amount of the Rep protein.
  • Plasmid requires the Rep protein to initiate the replication.
  • It is different from ColE1-derived plasmids which do not require Rep protein. Only the RNA primer was enough for it to start the replication process.
  • The Rep protein is required to separate the strands of DNA at the oriV region, often with the help of host proteins, including DnaA.
  • Opening the strands is a necessary first step of replication.
  • The Rep proteins bind specifically in the DSO of DNA
  • By controlling the synthesis of the Rep protein, the plasmid copy number can be controlled.
  • The R1 plasmid uses its complementary RNA to inhibit the translation of the mRNA that encodes the Rep protein and inhibits the replication of the plasmid DNA.
  • The plasmid-encoded protein RepA is the only plasmid-encoded protein that is required for the initiation of replication.
  • Two promoters transcribe the repA gene:
    • pcopB
    • pCopA

CopB:

  • CopB, codes for a protein that represses transcription of the repA gene.
  • When the plasmid first enters a bacterial cell, the absence of CopB allows expression of RepA and so there is a burst of replication until the level of CopB builds up to repress this promoter.
  • From then on, expression of RepA occurs at a low level from the copB promoter

CopA:

  • The second regulatory gene, copA, then regulates the expression of RepA.
  • This gene codes for an 80–90-nucleotide untranslated RNA molecule.
  • The copA gene is within the region of DNA that is transcribed for the production of RepA but is transcribed in the opposite direction (it is an antisense RNA).
  • The copA RNA is therefore complementary to a short region of the repA transcript and will bind to it, interfering with the translation of the RepA protein.
  • When the plasmid replicates, the number of copies of the copA gene is doubled and the amount of the copA RNA will therefore increase.
  • This causes a marked reduction in further replication initiation, until cell division
  • Copy number control of R1 plasmid
    Copy number control of R1 plasmid