Elongation factor-1a (EF-1a)
is a biochemical marker for lysine content in maize endosperm
--Moro, GL; Habben, JE; Carneiro, N; Hamaker, B and Larkins, B
We recently reported a very high correlation (r = 0.95**) between the content of lysine and the concentration of the protein synthesis factor EF-1a in the maize endosperm (Habben et al., PNAS 92: 8640-8644, 1995). In order to extend our analysis to a broader sample of the maize germplasm we characterized 93 normal and opaque2 inbred lines. Amounts of total protein, zeins and non-zeins were measured by microKjeldhal, and lysine content was determined by amino acid analysis. For twenty selected genotypes covering the observed range of lysine content an ELISA was used to estimate the relative concentration of EF-1a and a ninhy-drin assay was used to determine the relative levels of free amino acids. Considerable differences in lysine and protein contents were observed among normal and opaque2 genotypes, with the effect of the mutation being highly dependent on the genetic background. Not surprisingly, the lysine content was significantly correlated with the non-zein fraction (r = 0.83*** for all genotypes and r = 0.80*** for the selected lines). Most of endosperm lysine is protein-bound and, essentially, all the lysine-containing proteins are non-zeins. Confirming our previous results, a high correlation (r = 0.88***) was observed between EF-1a and lysine contents. It is remarkable that a single protein is at least as predictive of the lysine content as the total non-zein fraction. The nature of this relationship is still unknown. Although EF-1a is a lysine-rich protein its mass accounts for only 3-5% of the total lysine in the endosperm. Therefore, the high correlation must reflect some commonality between EF-1a and other lysine-rich proteins. We are now working on identifying such proteins. Independent of that, this relationship provides an approach to study the mechanisms regulating the synthesis and accumulation of lysine-rich proteins. We are also investigating the levels of heritability for EF-1a content, in order to assess its utility as an index for lysine content in breeding programs. Additionally, we are working on the characterization of the maize EF-1a gene family.
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