Ahm. Vandergeest et Tc. Hall, A 68 BP ELEMENT OF THE BETA-PHASEOLIN PROMOTER FUNCTIONS AS A SEED-SPECIFIC ENHANCER, Plant molecular biology, 32(4), 1996, pp. 579-588
In beans, expression of the beta-phaseolin gene (phas), encoding the m
ajor seed storage protein of bean (Phaseolus vulgaris) is confined to
the cotyledons of developing embryos. Phaseolin has not been detected
in the endosperm, which remains liquid and is lost early in developmen
t. However, fusion constructs between the phas promoter and the gus-co
ding region yield expression in both embryo and endosperm of developin
g seeds from transgenic tobacco (Nicotiana tabacum) plants. Although e
lements extending 1470 bp upstream of the transcription start site are
known to modulate phas expression, the proximal 295 bp (p295) are suf
ficient to drive high levels of seed-specific GUS activity. This regio
n was dissected into three elements: a 68 bp element (seed specific en
hancer, SSE: -295 to -227), a middle region (-227 to -109) and a basal
phas promoter(-109 to +20: p109). Different promoter constructs conta
ining the SSE or middle region upstream of p109 or a CaMV 35S basal pr
omoter (-64 to +6) were fused to gus. Each construct was expressed in
seed, but not in vegetative tissues. Use of the various phas promoter
regions yielded notable differences in relative GUS activity in embryo
or endosperm. Addition of both the SSE and middle region resulted in
higher activity than the sum of adding either element alone to p109, i
ndicating synergistic interaction between these elements. Seeds from p
lants transformed with the proximal 227 bp of promoter (p227) showed e
mbryo-specific GUS activity. In contrast, constructs containing two co
pies of the SSE element were preferentially expressed in the endosperm
. These results illustrate the modular nature of the proximal phas pro
moter, where distinct elements contribute to high levels of expression
in different parts of the seed.