Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

Table 11.3 MajorgenesinvolvedinthebiosynthesisofrhamnolipidsinPseudomonasaeruginosa[121].
Gene G + C %
promoter
researchers showed that rhlC is coordinately regulated with a rhlAB quorum-sensing
system. Major genes involved in this system are listed in Table 11.3.
The rhl system also regulates the stationary phase sigma factor encoded by rpoS,
which is involved in the regulation of numerous genes important for survival under
adverse conditions [116]. A second quorum-sensing system, located at a different
region in the P. aeruginosa chromosome, also influences the expression of rhamnolipid
biosynthesis. This second system is encoded by lasR (31% homology to rhlR)
and lasI (28% homology to rhlI) wherein lasI encodes the autoinducer N-(3-oxododecanoyl)-L-homo-serine
lactone [122]. The interaction between both systems was
described as a hierarchical quorum-sensing cascade, with lasR and lasI as the master
regulators [123].
Olvera et al. [124] proved that the algC gene, involved in alginate production
through its phosphomannomutase activity and in LPS synthesis through phosphoglucomutase
activity, participates in rhamnolipid biosynthesis in P. aeruginosa. The
phosphoglucomutase activity of AlgC is responsible for the production of glucose-1phosphate,
the precursor of dTDP-glucose and ultimately of dTDP-l-rhamnose,
whereas products of other alg genes are involved neither in rhamnolipid production
nor LPS synthesis. Pearson et al. [117] have shown that both quorum-sensing
systems can induce expression of rhamnolipids and other virulence genes but with
different efficiencies due to specificity of the transcriptional activator with its cognate
autoinducer.
11.4.3
Regulation
Size
(nt)
Peptide
length
Gene
product
(kDa)
pI Function
11.4 Rhamnolipidsj223
RhlA 65.8 54 887 296 32.5 7.4 Rhamnosyl- transferase 1
RhlB 67.9 54 1280 427 47 8.4 Rhamnosyl- transferase 1
RhlC 70.7 54 975 325 35.9 nd Rhamnosyl- transferase 2
RhlG Nd 54 768 256 26.8 nd NADPH-dependent ketoacyl reductase
RhlR 61.7 70 726 242 26.5 7.0 Transcriptional regulator
RhlI 64.8 64.8 NA NA NA NA NA
The synthesis of rhamnolipids in P. aeruginosa under nitrogen depletion, when the
cells shift into stationary growth phase, has been reported [125,126]. Ochsner et al.
[115] observed the expression of P. aeruginosa genes for rhamnolipid synthesis in
P. fluorescens and P. putida only under nitrogen-limited conditions. Addition of
nitrogen inhibits the production of rhamnolipids by resting cells in Pseudomonas
sp. DSM 2874 [102]. Guerra-Santos et al. [127] reported that higher levels of