Du3, a mRNA cap-binding protein gene, regulates amylose content in Japonica rice seeds

Abstract

Five non-allelic dull (du) mutants have been isolated and characterized as low amylose mutants in rice (Oryza sativa) endosperm. Synthesis of amylose is controlled by Waxy the gene that encodes granule-bound starch synthase. We previously showed that the du1 and du2 proteins may encode general splicing regulators which affect the expression level and splicing efficiency of Waxy^b (Wx^b), an allele that has a single-base mutation in the 5′ splice site of intron 1. Here we report that the du3 mutant has also reduced the splicing efficiency of Wx^b pre-mRNA. Furthermore, map-based cloning revealed that Du3 encodes the rice homolog of a cap-binding protein 20 kD subunit (CBP20), a component of the heterodimeric nuclear cap-binding complex (CBC) playing a role in pre-mRNA splicing, RNA nuclear export, and nonsense-mediated decay. Transient expression of green fluorescent protein (GFP)-tagged Du3 proteins in rice protoplasts resulted in localization to the nucleus showing a speckled pattern similar to the pattern displayed by other splicing factors.