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Functions of Ori

The ori region of plasmids helps determine their host range and controls replication. It pinpoints the site where replication begins. The ori region controls host range by allowing broad host range plasmids to replicate in many bacteria through promoters and ribosome sites that can be recognized across species. It also regulates copy number, determining how many plasmids are present per cell, and influences incompatibility, controlling whether two plasmids can stably coexist in the same cell based on shared replication or partitioning systems.

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Felix Simsung
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200 views8 pages

Functions of Ori

The ori region of plasmids helps determine their host range and controls replication. It pinpoints the site where replication begins. The ori region controls host range by allowing broad host range plasmids to replicate in many bacteria through promoters and ribosome sites that can be recognized across species. It also regulates copy number, determining how many plasmids are present per cell, and influences incompatibility, controlling whether two plasmids can stably coexist in the same cell based on shared replication or partitioning systems.

Uploaded by

Felix Simsung
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© © All Rights Reserved
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Functions of Ori Region

Ori region helps in determining the host range of the plasmids. This region is
extremely important as it pinpoints the site from which replication begins. The
Ori site of the plasmid controls or helps in determining the following functions:

A) Host Range

Host range refers to all types of bacteria within which a particular plasmid can
replicate. Broad host range plasmids are least dependent on their hosts as they
carry all the necessary genes and code for all the necessary proteins required for
their independent survival within distinctly related bacteria hence reflecting the
evolutionary divergence of these groups. Apparently the promoters and
ribosome initiation sites for the replication of these plasmids have eveloved so
that they can be recognized in a wide variety of bacteria.

B) Regulation of Copy Number

Another important characteristic of plasmids determined mostly by the ori site is


their copy number. Copy number refers to the average number of plasmids per
cell or more precisely the number of particular plasmids immediately after
division.
Plasmid copy number is strongly regulated other wise the cell would burden
itself with high copy numbers.
On the basis of copy number plasmids can be of the following two types;
Relaxed or High copy Number Plasmids
Stringent or Low Copy Number Plasmids
Study regulation of plasmid replication by complementary or
antisense RNA (page number 211-213 of the book Molecular
Genetics of Bacteria by Snyder)

C) Incompatibility

It refers to the ability of two plasmids to stably co-exist within the same cell.
Bacteria mostly contain multiple types of plasmids, the decision of whether
these plasmids will stay together within the cell or not is decided by
incompatibility groups that are actually dependent on the ori region. Plasmids
that can’t stably co-exist within one bacteria are referred to as belonging to the
same incompatibility group. If they can co-exist they are said to belong to
different incompatibility (Inc.) groups.
Incompatibility groups can be same due to either shared replication control or
either due to same partitioning system.
Plasmid Curing
The terminology as explained in the above figure refers to the loss of a plasmid from a bacterial
progeny due to any of the following reasons;

Resolution of Multimeric Plasmids


If during replication the plasmid forms dimers the chances of loss of a plasmid
are increased. A dimer consists of two copies of a same plasmid molecule linked
together from head to tail to form a large molecule and a larger circle. They
usually form due to recombination between two monomers. Sometimes rolling
circle replication if not properly terminated can also lead to the formation of
multimers. Multimers lower the copy number of the plasmids and while the cell
is dividing they might get be confused by the segregation machinery as a single
molecule and moved to one progeny rather then both hence curing one off spring
bacteria of the plasmid.
To avoid this plasmids have site-specific recombination systems that resolve
multimers as soon as they form. A site-specific recombination system promotes
recombination between specific homologous sites if they are repeated on the
same molecule more than once.
Partitioning System
Refer to page 227 and 228 of
Snyder for this topic

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