opaque2 modifiers act post-transcriptionally and in a polar manner on gamma-zein gene expression in maize endosperm.
The opaque2 (o2) modifier genes convert the soft endosperm of an o2 mutant to a hard, vitreous phenotype. The primary biochemical change associated with the expression of these genes is a two- to threefold increase in synthesis of the 27-kD gamma-zein storage protein. To investigate the mechanism of modifier gene activity, we examined the level of gamma-zein mRNA and protein synthesis during the early stages of endosperm development in normal, o2, and modified o2 geno-types. Although the o2 mutation was found to reduce expression of the 27-kD gamma-zein genes, the activity of o2 modifier genes dramatically increased the level of both gamma-zein protein and mRNAs as early as 16 days after pollination. At this stage, transcription of gamma-zein genes is reduced by approximately 50% in both o2 and modified o2 genotypes compared to wild type. Thus, it appears that the modifiers regulate gamma-zein synthesis through a post-transcriptional mechanism. Analysis of transcripts from the two nearly identical genes (A and B) encoding the 27-kD gamma-zein protein showed differences in the mRNA ratios in different genotypes. In modified o2 mutants, accumulation of A over B transcript was greatly enhanced during endosperm development. Somatic recombination at this locus was found to reduce the number of B genes in the endosperm, but this could not account for the preferential accumulation of the A transcript. Our results suggest that a product of the o2 modifier genes increases the translation or stability of the A gene mRNA, leading to enhanced synthesis of 27-kD gamma-zein protein.
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Department of Plant Sciences, University of Arizona, Tucson 85721.
Boyer
S K
SK
Larkins
B A
BA
D001483:Base Sequence; D005786:Gene Expression Regulation; D017343:Genes, Plant; D008969:Molecular Sequence Data; D009154:Mutation; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000378:metabolism; D012639:Seeds / Q000235:genetics; D014158:Transcription, Genetic; D003313:Zea mays / Q000235:genetics*; D015028:Zein / Q000235:genetics*
D012333:RNA, Messenger; D015028:Zein