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LEC2 Possesses a B3 Domain Characteristic of Transcription Factors.
Our findings that the LEC2 gene encodes a B3 domain protein
(Fig. 3) and is expressed primarily during embryogenesis (Fig. 4)
suggest that it, like LEC1 and FUS3, is a transcriptional regulator of
seed development. The B3 domain is an ≈120-amino acid residue region
defined originally as the third basic region of maize VP1, which shares
the largest contiguous block of sequence identity with its ortholog,
Arabidopsis ABI3 (10). Subsequently, this domain was
identified in other plant proteins, such as AUXIN RESPONSE FACTOR1 (34)
and RELATED TO ABI3/VP1 (32). Many proteins containing B3 domains,
including ABI3/VP1, FUS3, and AUXIN RESPONSE FACTOR1, function as
transcription factors (20, 33, 34). The B3 domain is responsible, at
least in part, for the DNA-binding activity of ABI3/VP1 and RELATED
TO ABI3/VP1 (32, 37, 38). To our knowledge, the B3 DNA-binding domain
is unique to plants.
Maximum parsimony analysis (39) of the B3 domains of 48 predicted
Arabidopsis proteins suggests that LEC2, FUS3, and ABI3
constitute one class, perhaps indicating their common origin (S.L.S.
and J.J.H., unpublished results). All three genes are expressed
primarily during embryogenesis and are required genetically for the
completion of normal seed development. Although they all play roles in
seed development, their specific functions have diverged. ABI3 is a
transcription factor that operates primarily during the maturation
phase, whereas LEC2 and FUS3 are LEAFY COTYLEDON proteins required for
processes during both the morphogenesis and maturation phases.
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