Plasmids are
present in all branches of the bacteria ‘tree of life’ and have been found in
all bacterial communities studied to date, including soil, and marine and environments.
However, only a limited amount of plasmid-sequence data is currently available (Mølbak et al.,
2003).

Plasmids are important genetic engineering tools and
the vectors of horizontal gene transfer that may harbor genes involved in
virulence and antibiotic resistance. So, the studying of plasmids is important
for understanding the evolution of these traits and for tracing the
proliferation of drug-resistant bacteria and also to understand the epidemiology
of plasmids (Antipov et al., 2016). Moreover, they provide a means to clone,
complement,express host genes, and to introduce selected genes into different
genetic backgrounds (Pashley and Stoker, 2000).

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Plasmids have been widely reported among many
mycobacterial species (Pashley and
Stoker, 2000) from both clinical and environmental sources often contain naturally
occurring plasmids of varying size (10 to >100 kb) ( Falkinham
and Crawford, 1994; Pashley and Stoker, 2000) and
some isolates in some cases carry multiple plasmid (Crawford et al., 1981).

There is very few information about the function of
these mycobacterial plasmids, although several studies have suggested that
genes involved in different forms of hydrocarbon metabolism are plasmid borne (Coleman and Spain, 2003; Guerin and Jones,
1988).

The widespread occurrence of
related plasmids from different isolates suggests that the plasmids can
transfer between mycobacterial species in the environment. Little is known
about these plasmids, their mechanism of replication and coexistence, the genes
that they encode or whether the plasmids play any role in virulence. Various
reports have speculated that they may contribute to virulence and may encode
resistance to metals, restriction modification systems and metabolic enzymes (Crawford et al., 1981; Meissner and Falkinham,
1984; Erardi et al., 1987).

However, the inability to cure
plasmids from these isolates, as well as the lack of selectable markers and an
understanding of the genetic makeup of these MAC plasmids, have prevented both
rigorous analysis of their genetic composition and confirmation of plasmid
associated functions (Falkinham and Crawford, 1994; Pashley and Stoker,
2000).

Crawford and colleagues were the first to identify
plasmids in M. avium (Crawford,
1979) and five years later they characterized a 15.3 Kb plasmid, pLR7 (Beggs, 1995). M. avium’s
plasmid, pLR7, is very simple and is known to harbor genes encoding a Rep
protein and a surface-associated protein (Beggs,
1995).

Plasmids have also been found in M. scrofulaceum,
M. chelonae, and M. abscessus species and subspecies (Bachrach, 2000;Gavigan, 1997;Labidi, 1984;Labidi, 1992;Meissner, 1984).

The Rep regions of several of the plasmids identified
in mycobacteria have been

sequenced and show high degrees of sequence homology
with each other (Gavigan, 1997).

Scientists are investigating the host range of these
plasmids to assess their potential use in mycobacterial transformations and heterologous
expression of genes (Bachrach, 2000).

Nucleotide sequencing has enabled a broader
understanding of mycobacterial plasmids. The ability to compare the nucleotide
similarity of the rep regions of various mycobacterial plasmids may provide
useful insight into their host range. For example, the repA gene from
the M. ulcerans plasmid, pMUM001, shares greater than 68% amino acid
identity with rep from the M. fortuitum plasmid pJAZ38 and 55.6%
amino acid identity with the same rep gene in pVT2, a plasmid harbored
by M. avium (Stinear, 2005).

In general, mycobacterial plasmids have only been
partially characterized, while the focus has been their development as vectors
(Movahedzadeh and bitter 2009, Pashely
and stoker 2000).

Plasmids
have a dominant role in the horizontal transfer of genetic information between
bacteria and can transfer DNA between genera, phyla and even major domains (Turner
et
al.,
2002; van Elsas et al.,2002)
by a mechanism that is known as bacterial conjugation.

Conjugative
plasmids are a powerful tool for genome evolution as they can harbor and
transfer genes between organisms sampling all genomes within an ecosystem (Norman
et al., 2009).

Traditionally, plasmid transfer is detected by growth of transconjugants on
selective media, although molecular methods also are available. Selective
growth of transconjugants requires that the plasmids are stable for several
generations in the recipient species. This is a drawback, since recombination
events between the plasmid and the host genome may occur even if the plasmid is
not stably maintained. New methods for detection of gene transfer have recently
been designed based on the expression of lacZ (Jaenecke et al. 1996) and gfp (Christensen,
Sternberg, and Molin 1996).

A conjugative plasmid called pRAW was detected for the first time by Ummels et al., 2014, this plasmid has a very big size which reach to 114.229 and this plasmid can
transfer to slow growing mycobacterium and so the plasmid has threat and
opportunity. The threat is that, through the acquisition of antibiotic
resistance markers, this plasmid could start a rapid spread of antibiotic
resistance genes between pathogenic mycobacteriaand the opportunity is through
generate new tool for introduction foregin DNA in slow growing mycobacterium.