The expression of
alpha-amylase genes in cereals is induced by both
gibberellin (GA) and
sugar starvation. All
alpha-amylase genes isolated from cereals contain a TATCCA
element or its variants at positions approximately 90 to 150 bp upstream of the transcription start sites. The TATCCA
element was shown previously to be an important component of the GA response complex and the
sugar response complex of
alpha-amylase gene promoters. In the present study, three
cDNA clones encoding novel MYB
proteins with single
DNA binding domains were isolated from a rice
suspension cell cDNA library and designated OsMYBS1, OsMYBS2, and OsMYBS3. Gel mobility shift experiments with OsMYBSs showed that they bind specifically to the TATCCA
element in vitro. Yeast one-hybrid experiments demonstrated that OsMYBS1 and OsMYBS2 bind to the TATCCA
element and transactivate a promoter containing the TATCCA
element in vivo. Transient expression assays with barley half-seeds showed that OsMYBS1 and OsMYBS2 transactivate a promoter containing the TATCCA
element when
sugar is provided, whereas OsMYBS3 represses transcription of the same promoter under
sugar starvation. Transient expression assays also showed that these three OsMYBSs cooperate with a GA-regulated
transcription factor, HvMYBGa, in the transactivation of a low-pI barley
alpha-amylase gene promoter in the absence of GA. Two-hybrid experiments with barley half-seeds showed that OsMYBS1 is able to form a homodimer. The present study demonstrates that differential
DNA binding affinity, promoter transactivation ability, dimerization, and interactions with other
protein factors determine the biological function of OsMYBSs. This study also suggests that common
transcription factors are involved in the
sugar and hormonal regulation of
alpha-amylase gene expression in cereals.