5 The role of quorum-sensing in the virulence of Pseudomonas ...

having multiple copies of itself. An important feature of IS1 is it ability to excise
from the genomic DNA and circularise (Sekine et al. 1997). IS3, the first 71bp of
total sequence length of 1258bp is found; none of the two ORFs are found within
this small region; but IS3 might have been important in the early changes in the
genomic island structure. The 3’-end of leuX still have its core sequence (attB) is
intact and therefore allows further acquisition of other P4 family prophages and
could add to the current genomic islet in a modular fashion.
The evolution of the genomic islet, E105-leuX seems to have progressed originally
from a lysogenic bacteriophage that integrated into the leuX tRNA site. This
bacteriophage acquired a mutation in its excision gene; this left the bacteriophage
unable to propagate. A research paper by Campbell (2003) supports this notion. It
suggests that a genomic island started as a bacteriophage that originally integrate
into a tRNA site, but become defective. The genome of the bacteriophage is then
subjected to selective pressure to lose bacteriophage genes non-essential to the
bacterial cell, but the site continued to collect other foreign DNA. The only remnant
defined as a bacteriophage is the integrase (int) gene but lacks any other markers to
enable identification of the original bacteriophage (Campbell 2003). In the case of
E105-leuX this would suggest that the defective prophage underwent selection to
maintain the type III RM system but promoted the loss of any essential genes for
viral propagation such as those required to produce a viral capsid. Interestingly, the
genes for IS1 and the type III RM system are found in the same orientation to each
other in the genome of Enterobacteria phage P1; separated by essential viral genes
such for the capsid, which would not be needed by the bacterium. This adds strength
to assumption that the DNA sequence of E105-leuX is a remnant of similar events.
The integrase has also become non-functional as it no longer needed by the
prophage. Remnants of insertion sequences that have aided the evolution are left
behind also such as IS3.
E105-leuX is a novel genomic islet discovered within clinical isolate E. coli E105
that was characterised within this section. Analysis of the DNA sequence suggests
that the genomic island is a remnant of larger genomic island that appeared to
encode a number of functional restriction endonucleases. The genomic island
functioned to protect the bacteria from bacteriophage infection. This work also
85