Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 68

archaeal species (Burrus et al. 2002 ; Burrus and Waldor 2004 ; Juhas et al. 2007 ,
2008 ; Alvarez-Martinez and Christie 2009 ). These elements are excised from the
chromosome through the action of a recombinase/excisionase complex and
followed by the formation of a circular intermediate. Then the circularized inter-
mediate is processed at oriT in the same way as described for conjugative plasmids.
In the recipient cell, ICEs reintegrate into the chromosome (or plasmid) by homol-
ogous recombination or through the action of an integrase encoded by the ICE itself
(Alvarez-Martinez and Christie 2009 ). Conjugative plasmids and ICEs are recruited
to the transfer machine through interactions between the relaxosome or processed
DNA transfer intermediate and a highly conserved ATPase termed the type IV
coupling protein. This protein interacts with the translocation channel, which
consists of the mating-pair formation proteins (Christie 2004 ; Schr¨der and
Lanka 2005 ; Alvarez-Martinez and Christie 2009 ). Two types of mating-pair
formation proteins, an ATPase and a polytopic membrane subunit, are associated
with all T4SSs, whereas other mating-pair formation proteins are less phylogenet-
ically conserved. In Gram-negative bacteria, the mating-pair formation proteins
build the secretion channel, as well as a pilus or other surface filaments, to achieve
attachment to target cells (Lawley et al. 2003 ; Christie and Cascales 2005 ; Alvarez-
Martinez and Christie 2009 ). In Gram-positive bacteria, surface adhesins rather
than conjugative pili apparently mediate attachment (Grohmann et al. 2003 ;
Alvarez-Martinez and Christie 2009 ). For the majority of Gram-positive bacteria
the origin and nature of the surface adhesins or other surface located factors
involved in attachment and/or recognition of the recipient cell have not been
elucidated so far.
2.2 DNA Uptake via Transformation
DNA transformation is based on the uptake of free DNA from the environment and
therefore does not rely on MGEs; it is only encoded by the acceptor bacterium.
Natural competence is the developmental state of the bacterium in which it is
capable of taking up external DNA and to recombine this DNA into the chromo-
some, thereby undergoing natural transformation (Seitz and Blokesch 2013 ). A
wide variety of bacterial species can develop natural competence and consequently
take up external DNA (for recent reviews, see Lorenz and Wackernagel 1994 ; Chen
and Dubnau 2004 ). The main steps to uptake external DNA include (1) binding of
double-stranded DNA outside the cell to a (pseudo-) pilus structure elaborated by
the acceptor cell, (2) extension and retraction of the pilus, driven by ATP-dependent
motor proteins, that mediate the uptake of the double-stranded DNA through the
secretin pore, which spans the outer membrane of the acceptor cell, (3) binding of
the double-stranded DNA by the DNA-binding protein ComEA which occurs in the
periplasmic space, (4) transport across the inner membrane which is carried out by
ComEC concomitantly with the degradation of one strand by an unidentified
nuclease, (5) single-stranded DNA reaches the cytoplasm and is decorated by