nodgenes to synthesize LCO. It might be noted
that theLotuscompounds that activate NodD for
induction ofnodgenes have not been determined
yet, though aldonic acids have been reported to
induceM. loti nodgenes at high concentration
(Gagnon and Ibrahim 1998 ).
The predominant LCO species produced by
M. loti strains MAFF303099 and R7A is an
N-acetylglucosamine pentasaccharide. The non-
reducing residue is N-methylated and N-acylated
with cis-vaccenic acid (C18:1) or stearic acid
(C18:0) and carries a carbamoyl group, and the
reducing terminal residue carries a 4-O-acety-
lfucosyl residue (Lopez-Lara et al. 1995 ; Niwa
et al. 2001 ; Rodpothong et al. 2009 ). The core
N-acetylglucosamine oligomer is synthesized by
thenodCgene product,N-acetyl-glucosaminyl-
transferase, that extends the oligomer at the non-
reducing terminus (Geremia et al. 1994 ). The
nodBgene encodes a deacetylase that removes
theN-acetyl moiety from the assembledN-ace-
tylglucosamine oligomer at the non-reducing
end (John et al. 1993 ). An acyl chain is then
linked to the deacetylated oligomer by thenodA
gene product (Rohrig et al. 1994 ). Although
these assembly reactions are common and the
genes shared in all rhizobia, the number of
oligomeric repeats and the nature of added acyl
chains are determined in part by the substrate
specificity of the NodC and NodA proteins
(Ritsema et al. 1996 ; Roche et al. 1996 ; Kamst
et al. 1995 ).
Most othernodgenes participate in specific
modifications of the core structure. The nodS
and nolO genes flanking nodACIJ encode
methyltransferase and carbamoyltransferase,
respectively. ThenolLgene encodes O-acetyl-
transferase, whilenodMis a glutamine-fructose-
6-phosphate transaminase. Gene products of
noeK–noeJandnoeL–nolKoperons participate
in the synthesis of GDP-D-mannose and GDP-
L-fucose from GDP-D-mannose, respectively,
while thenodZgene encodes a fucosyltransfer-
ase. The assembled LCO is probably secreted by
the function of NodI and NodJ proteins (Spaink
1995 ). Mutagenesis ofnodLandnodZgenes in
R7A revealed that theN-acetyl-fucose modifi-
cation(s) of the LCO affects symbiotic perfor-
mance depending on hostLotusspecies, whereas
the methylation and carbamoylation apparently
had no effect (Rodpothong et al. 2009 ).
Although not involved in Nod factor synthesis,
the geneacdS, encoding 1-aminocyclopropane-
1-carboxylic acid (ACC) deaminase, is conserved
in the three symbiosis islands. The product pos-
sibly perturbs host ethylene synthesis in order to
enhance nodulation capacity (Uchiumi et al. 2004 ;
Nukui et al. 2006 ).Fig. 5.2 Comparison of threeM. lotisymbiosis islands at nucleotide level by Murasaki (Popendorf et al. 2010 )
5 Genome Sequence and Gene Functions... 47