--Jayne L. Puckett, Mark Wallace and Alan L. Kriz
Globulins are the most abundant proteins in maize embryos. Previous
studies have demonstrated that globulins are also present in the endosperm
and that the amounts of these proteins are increased in opaque-2 endosperm
relative to normal endosperm (Dierks-Ventling and Ventling, FEBS Lett.
144:167). To determine if similar changes in globulin constitution occur
in the embryo, and whether such changes are a result of zein deficiency,
we investigated globulin gene expression in kernels of various genotypes.
Specifically, we compared normal kernels with kernels homozygous for the
opaque-2 and floury-2 mutations in each of W64A, Oh43, and W22 backgrounds.
Total embryo and endosperm proteins from mature kernels were analyzed by
SDS-PAGE and by immunoblot analysis in which antiserum specific to the
Glb1 proteins was used as probe. The amount of Glb1-specific
proteins was two to four times higher in both the embryo and endosperm
of opaque-2 kernels, relative to levels in the respective normal tissue,
in each of the three backgrounds. Northern blot analysis of embryo RNA
indicates the amount of Glb1 transcript in opaque-2 embryos is at
least twice that in normal embryos. No increases in amount of Glb1
proteins were observed in floury-2 kernels. In addition, no changes in
the level of the Mr 45,000 Glb2 protein were found in any of the
genotypes examined. These data suggest the following: 1) of genes encoding
globulins, Glb1 expression is specifically increased in opaque-2
kernels and this increase occurs at the transcriptional level; and 2) this
increase does not appear to be due to decreased zein synthesis since the
zein-deficient floury-2 kernels contain normal amounts of Glb1 proteins.
Since the Glb1 proteins contain a significant amount of lysine (5.6
mole percent), the elevated lysine content in opaque-2 kernels may be due
in part to increased amounts of these proteins.
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