ColE1 is a

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...

found in

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

. Its name derives from the fact that it carries a gene for

colicin A colicin is a type of bacteriocin produced by and toxic to some strains of ''Escherichia coli''. Colicins are released into the environment to reduce competition from other bacterial strains. Colicins bind to outer membrane receptors, using them ...

E1 (the ''cea'' gene). It also codes for immunity from this product with the ''imm'' gene. In addition, the plasmid has a series of mobility (''mob'') genes. Its size and the presence of a single EcoRI

recognition site A recognition sequence is a DNA sequence to which a structural motif of a DNA-binding domain exhibits binding specificity. Recognition sequences are palindromes. The transcription factor Sp1 for example, binds the sequences 5'-(G/T)GGGCGG(G/A)( ...

caused it to be considered as a

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

candidate.

Replication

ColE1 replication begins at the

origin Origin(s) or The Origin may refer to: Arts, entertainment, and media Comics and manga * Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002 * The Origin (Buffy comic), ''The Origin'' (Bu ...

. 555 bp upstream from this point, RNA polymerase initiates transcription of RNAII which acts as a pre-primer and begins the synthesis of the leader strand. The transcript folds into a secondary structure which stabilises the interaction between the nascent RNA and the origin's DNA. This hybrid is attacked by

RNase H Ribonuclease H (abbreviated RNase H or RNH) is a family of non-sequence-specific endonuclease enzymes that catalyze the cleavage of RNA in an RNA/ DNA substrate via a hydrolytic mechanism. Members of the RNase H family can be found in nearly ...

, which cleaves the RNA strand, exposing a 3' hydroxyl group. This allows the extension of the leading strand by

DNA polymerase I DNA polymerase I (or Pol I) is an enzyme that participates in the process of prokaryotic DNA replication. Discovered by Arthur Kornberg in 1956, it was the first known DNA polymerase (and the first known of any kind of polymerase). It was initi ...

. Lagging strand synthesis is later initiated by a primase encoded by the host cell.1 Replication is carried out entirely by host proteins (RNA polymerase, DNA polymerase I and RNase H) so that inhibition of translation will stop the growth of the cells, but not the replication of ColE1. Since the translation of Rop protein will also be inhibited, a higher than normal copy number will result in these cells.

Copy number control

RNAI RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by o ...

is a counter-transcript to a section of RNAII and so binds to its 5' end. This alters the folding of RNAII so that the DNA-RNA hybrid is not stabilized and cleavage does not occur. This ensures that at high

copy number Copy number variation (CNV) is a phenomenon in which sections of the genome are repeated and the number of repeats in the genome varies between individuals. Copy number variation is a type of structural variation: specifically, it is a type of d ...

s, replication is slowed down due to increased RNAI concentration. ''Rop'' is a secondary replication repressor, it stabilizes the RNAI-RNAII hybrid. Rop may be especially important at preventing runaway replication at slow growth rates. ColE1 replication control

ColE1 has a copy number of 25-30 according to its original publication

References

{{reflist, 2 Mobile genetic elements Bacteriology Plasmids