majority of theM. lotipopulation by QseM, a
recently identified protein antiactivator of TraR.
QseM expression is in turn regulated by the
DNA-binding protein QseC, which binds and
represses theqseMpromoter in a concentration-
dependent manner. This complex regulation may
form a molecular switch allowing activation of
quorum sensing, excision, and transfer in a small
proportion of cells in the population, rather than
the population-wide activation typical of many
quorum-sensing systems (Ramsay et al. 2009 ,
2013 ). This novel regulation mechanism with
qseM–qseCseems to be conserved among later-
ally transferable integrative and conjugative ele-
ments that are found in various bacteria, although
most of these elements do not contain TraR
homologues (Ramsay et al. 2013 ).
5.6 Genetic Resources
A collection of transposon mutants of
MAFF303099 was constructed by Shimoda and
colleagues (Shimoda et al. 2008 ). Using the
signature-tagged mutagenesis (STM) technique,
more than 20,000 mutants were generated.
Determination of insertion positions in more than
7,800 mutants identified insertions into approxi-
mately 50 % of MAFF303099 genes. These
mutants with defined insertion points are depos-
ited with the National BioResource Project (L.
japonicusandG. max) in Japan and accessible
from the Web site (http://www.legumebase.brc.
miyazaki-u.ac.jp). Also available at the National
BioResource Project (L. japonicusandG. max)
in Japan are plasmid clones of defined inserts
obtained during the sequencing project of the
strain MAFF303099 (Kaneko et al. 2000 ).
5.7 Concluding Remark and Future
Prospects
Here, we treatedM. lotias a collective name for
mesorhizobial species that nodulateLotusspe-
cies. Ever accumulating data on the species
indicate that the variousM. lotistrains have been
generated through lateral integration of symbiosis
islands of probably a single origin into the core
chromosome of a range of bacteria belonging to
Phyllobacteriaceae. TheM. lotisymbiosis islands
are evolving by acquiring (and losing) insertional
elements while maintaining conserved gene sets
for nodulation, nitrogenfixation, and other sup-
portive processes. Acquisition of new elements
on the islands does not necessarily enhance the
symbiotic capacity, but rather often restrict the
capacity as exemplified by the nodulation
restrictions withL. halophilusandL. leucocep-
hala, respectively, by possession ofmlr6361in
MAFF303099 andmsi059in R7A (Hubber et al.
2004 ; Okazaki et al. 2010 ). Although it is still
obscure how the combination of core chromo-
some and the island determines the nodulation
and nitrogen-fixing efficiency and host range of a
Lotusrhizobium, the peculiar gene regulation for
nitrogenfixation, substrate metabolism as well as
surface or extracellular components determined
by the core chromosome might restrict the com-
binations. With the rapidly advancing sequencing
technology and more traditional molecular
genetic analysis, the genomic basis of the
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