--Christian Carson and Donald McCarty
Null mutant alleles of vp1 produce viviparous, anthocyanin deficient kernels. The structure and expression of dormant vp1 alleles was determined and a summary is presented in the table. Leaky expression is exhibited by the vp1-mum3 allele; the level of anthocyanin expression is comparable to the apparent level of vivipary. Alternatively, full expression of the dormant, colorless alleles effectively promotes maturation, but not anthocyanins.
The maturation-related genes, Glb1 and Em, and the anthocyanin regulatory gene, C1, are activated by Vp1. Expression of Glb1 and Em mRNA is relatively normal in vp1-McWhirter and vp1-c821708 embryos; and consistent with the dormant, colorless phenotype, C1 expression is undetectable.
vp1-McWhirter mRNA is 3' truncated and the cDNA sequence encodes
a C-terminal truncated protein. vp1-Mc protein, immunoprecipitated
from in vitro translation and resolved on SDS-PAGE, is proportionately
smaller than wildtype. In fact, all of the dormant, colorless alleles encode
similarly sized, apparently truncated protein. In contrast, vp1-mum3
expresses normal sized message and protein.
|kernels||Glb1*||Em*||C1*||mRNA (bases)||Protein (~M in kDa)|
|6 vp1-Ref. (null)||v/c||-||-||-||none|
We have compared vp1-Mc with wildtype for activation of both the Em and C1 promoters during transient expression in electroporated maize protoplasts. The result is again consistent with the phenotype. In summary, vp1-Mc activates Em-GUS expression 10-20 fold, about 5% of wildtype. C1-GUS is activated 6 fold by wildtype, but it is not activated by vp1-Mc.
We conclude that the Vp1 gene is composed of at least two distinct
functional domains. The N-terminal portion of the protein, which is encoded
mostly by the first exon, is sufficient in activating maturation and the
promoter, while the C-terminal structure is required additionally for the
activation of C1.
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