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includes homologs of hcp1, IcmF, and clpV (as VIMSS583005, 582995, and 583009,
respectively, in D. aromatica ). This IcmF-associated (IAHP) cluster has been associ-
ated with mediation of host interactions, via export of effector proteins that lack signal
sequences [53]. Further evidence for type VI secretion is found in the presence of
three proteins containing a Vgr secretion motif modeled by TIGRfam3361, which is
found only in genomes having type VI secretory apparatus. Though most bacteria that
contain IcmF clusters are pathogenic agents that associate with eukaryotic cell hosts
[54], it has been reported that the host interactions supported by this cluster are not
restricted to pathogens [55].
The type IV pili systems might be involved in biofi lm development, as interactions
with biofi lm surfaces are affected by force-generating motility structures, including
type IV pili and fl agella [56]. Quorum sensing is a deciding input for biofi lm forma-
tion, and the presence of an eps synthetic cluster lends further support for biofi lm
formation. Further, derivatives of NO, which is an evident substrate for D. aromatica ,
are a key signal for biofi lm formation versus cell dispersion in the microbe P. aerugi-
nosa [57].
Cellular Interactions with Community—Quorum Sensing
Quorum sensing uses specific membrane-bound receptors to detect autoinducers re-
leased into the environment. It is involved in both intra- and inter-species density
detection [58, 59]. Cell density has been shown to regulate a number of cellular re-
sponses, including bioluminescence, swarming, expression of virulence factors, secre-
tion, and motility (as reviewed in Withers et al., 2001 [60]).
Dechloromonas aromatica encodes six histidine kinase receptor proteins that
are similar to the quorum sensing protein QseC of E. coli (VIMSS580745, 582451,
582897, 583274, 3337577, (formerly 583538), and 583893), fi ve of which co-occur
on the chromosome with homologs of the CheY like QseB regulator, and two of which
appear to be the product of a recent duplication event (VIMSS583893 and 3337577).
Of the six QseC homologs, phylogenetic analysis indicates VIMSS582451 is most
similar to QseC from E. coli , where the QseBC complex regulates motility via the Fl-
hCD master fl agellar regulators (VIMSS582640 and 582641). The presence of several
qseC/B gene pairs indicates the possibility of specifi c responses that are dependent on
different sensing strategies. In other species, expression of ABC exporters is regulated
by quorum sensing systems [46]; gene family expansion is indicated in the ABC ex-
port gene pool as well as the qseC/B sensors in D. aromatica .
N-acyl-homoserine lactone is the autoinducer typical for gram negative bacteria
[61], yet D. aromatica lacks any recognizable AHL synthesis genes. Ralstonia Beta-
proteobacteria likewise encode several proteins in the qseC gene family and display
a diversity of candidate cell density signaling compounds other than AHL [62]. The
utility of having a diverse array of quorum sensing proteins remains to be determined,
but appears likely to be associated with a complex, and possibly symbiotic, lifestyle
for D. aromatica .
 
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