Horizontal components involved in DNA-uptake are not the

Horizontal  gene transfer (HGT) is the mechanism through
which genomic islands move between bacteria. The three major HGT mechanisms are
conjugation (gene exchange through plasmids), transformation (gene exchange
through extracellular DNA) and transduction (virus or phages transferring the


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The term
transformation refers to the process of HGT in which DNA uptake from the
environment occurs (6,7). The ability to uptake exogenous DNA is known as
“natural competence”. In bacteria, natural competence is a complex process that
requires the expression of genes involved in the assembly of type IV pili and
type II secretion systems (6,8). Expression of these sets of genes (about 40
genes in Bacillus subtilis) depends on specific physiological and environmental
cues such as high cell density and limited nutrient availability . Certain
bacterial species, such as Neisseria gonorrhoeae and Haemophilus infuenzae, are
always competent to accept DNA, whereas others, such as Bacillus subtilis and
Streptococcus pneumoniae, become competent upon reaching a certain
physiological stage in their life cycle(1,7).

A large
and versatile family of Type4 Secretion System-dependent transport systems
contains conjugation systems 32-33. These are encoded by multiple genes which
are organized into a single operon. according to the organization of genetic
determinants, shared homologues and evolutionary relationships,type 4 secretion
system can be classified into several types 34. T4SSs have a role in the
horizontal transfer of a wide variety of genomic islands in a broad spectrum of
bacteria, including Haemophilus spp., Pseudomonas spp., and S. enterica serovar
Typhi. 34-36. Genes necessary for formation of a conjugative pilus and other
proteins necessary for the island transfer are encoded by the T4SS of ICE Hin
1056 35. A better explanation of how GEIs can propagate and efficiently enable
bacterial population to adapt to rapidly changing environments can be provided
by the presence of a highly evolved and efficient conjugation system for
mobilizin ggenomic islands.

components involved in DNA-uptake are not the same for gram-positive and
gram-negative bacteria due to the difference in cell wall structure. In
gram-positive bacteria, retraction of a pseudopilus opens a cell wall hole that
allows DNA to diffuse from the surface. In gram-negative bacteria, due to the
presence of an extra membrane, DNA uptake requires the presence of a more
complex channel, mainly formed by secretins (PilQ). In contrast to DNA uptake,
DNA translocation across the cell membrane is similar in gram-negative and
gram-positive bacteria. In both groups, homologues of the ComEC channel
proteins mediate the transport of the DNA to the cytoplasm. During this
process, one strand of the incoming DNA is degraded by nucleases, and the
remaining single-stranded DNA is bound by proteins that protect it from degradation.
Incorporation into the chromosome can be catalyzed by the mechanisms of
homologus recombination if sufficient sequence identity exists(2,6).