New Insights into Desiccation-Associated Gene Regulation by Lilium longiflorum ASR during Pollen Maturation and in Transgenic Arabidopsis - International Review of Cell and Molecular Biology - page 46

from a combination between tandem duplication and whole genome dupli-
cation events, and that their differential regulation under water stress prob-
ably involves subfunctionalization ( Philippe et al., 2010 ).
The polymorphism of Asr genes can also be extended to allelic diversity.
The grape VvMSA gene exhibits allelic polymorphism ( Saumonneau et al.,
2012 ) based on the BLAST analysis of the VvMSA gene sequence against
the proteome 8× deduced from the Pinot noir genome ( Jaillon et al., 2007 ).
The mAsr3 and OsAsr3 genes exhibit the most sequence variation among
the cultivars tested in banana and rice ( Henry et al., 2011 ; Philippe et al.,
2010 ). OsAsr3 is the most divergent of the rice Asr genes and exhibits
overall neutral selection at the species level, but directional selection in the
japonica subgroup found in tropical regions ( Philippe et al., 2010 ). Similar
to mAsr3 , the presence of polymorphic variants of the water stress-induced
Asr2 gene of tomatoes undergoes adaptive changes to possibly adapt to arid
habitats ( Frankel et al., 2003 ; Giombini et al., 2009 ). By contrast, Fischer
et al. (2011) suggested that the tomato Asr4 gene, but not Asr2 , shows pat-
terns consistent with local adaptation living in an extremely dry environ-
ment, whereas Asr1 has evolved under strong purifying selection.
3.2. ASR Protein
3.2.1. Structural Features
The lily LLA23 protein is rich in Glx and Gly, a characteristic of heat-sta-
ble proteins ( Wang et al., 1996 ). The predicted molecular mass of LLA23
is around 16 kDa; however, the protein ran at 23 kDa when fractionated
under SDS-polyacrylamide gel electrophoresis ( Wang et al., 1996 ). This high
apparent molecular mass is caused by a highly hydrophilic property, which
is often characteristic of intrinsically unstructured proteins ( Tompa, 2002 ).
The protein contains two main highly conserved regions, a small N-terminal
consensus region containing a stretch of six His residues and a large con-
sensus region at the C-terminus. The six His residues at the N-terminal
consensus may bind with zinc ions. Goldgur et al. (2007) suggested that a
pair of zinc ions tightly bound with ASR1 induces homodimer formation.
A dimeric protein structure is a commonly observed motif among DNA-
binding proteins ( Burley and Kamada, 2002 ). In the C-terminal consensus
sequence, it possesses two ABA/WDS signature sequences ( Canel et al., 1995 ;
Padmanabhan et al., 1997 ) and a putative nuclear localization signal (NLS)
sequence at the end of the molecule ( Wang et al., 2005 ). The bipartite NLS
motif of the lily ASR, KK TL KK ENEEVEG KK , has two functional clusters
of basic amino acids separated by a spacer of several nonconserved residues.