Mutations Affecting Light Regulation of Nuclear Genes Encoding Chloroplast Glyceraldehyde-3-Phosphate Dehydrogenase in Arabidopsis

Abstract

Expression of nuclear genes that encode the A and B subunits of chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPA and GAPB) of Arabidopsis is known to be regulated by light. We used a negative selection approach to isolate mutants that were defective in light-regulated expression of the GAPA gene. Two dominant mutants belonging to the same complementation group, uga1-1 and uga1-2, were then characterized. These two mutants showed a dramatic reduction in GAPA mRNA level in both mature plants and seedlings. Surprisingly, mutations in uga1-1 and uga1-2 had no effect on the expression of GAPB and several other light-regulated genes. In addition, we found that the chloroplast glyceraldehyde-3-phosphate dehydrogenase enzyme activity of the mutants was only slightly lower than that of the wild type. Western-blot analysis showed that the GAPA protein level was nearly indistinguishable between the wild-type and the uga mutants. These results suggested that posttranscriptional control was involved in the up-regulation of the GAPA protein in the mutants. The uga1-1 mutation was mapped to the bottom arm of chromosome V of the Arabidopsis genome.Transcription is one of the primary steps at which light regulates gene expression in plants (Terzaghi and Cashmore, 1995). Two classes of photoreceptors, phytochrome and blue light/UV-A receptor (cryptochrome), are involved in the regulation of photosynthetic genes (Batschauer, 1998; Briggs and Huala, 1999; Deng and Quail, 1999; Fankhauser and Chory, 1999). It has been suggested that eukaryotic phytochromes are Ser/Thr kinases with a two-component His kinase ancestry (Yeh et al., 1997;Yeh and Lagarias, 1998; Fankhauser and Chory, 1999; Fankhauser et al., 1999). Five phytochrome genes have been identified in Arabidopsis (Clack et al., 1994;Quail et al., 1995;Quail, 1997). Current evidence indicates that the different phytochromes may have distinct functions (Quail et al., 1995;Quail, 1997). Genetic and molecular studies have led to the identification of four blue-light photoreceptors in Arabidopsis (Briggs et al., 2001). CRY1 (HY4) and CRY2/PHH1 have partial overlapping functions in promoting anthocyanin formation and inhibiting hypocotyl elongation (Ahmad and Cashmore, 1993; Ahmad et al., 1995; Lin, 2000), whereas PHOT1/NPH1 and PHOT2 regulate phototropism, stomatal opening, and chloroplast movement (Liscum and Briggs, 1995; Briggs and Huala, 1999; Kinoshita et al., 2001;Sakai et al., 2001). In addition, the mutations in CRY1 and CRY2 genes affect blue-light-mediated regulation of photosynthetic gene expression (Ahmad et al., 1995; Conley and Shih, 1995; Mazzella et al., 2001).Several mutants affecting the light signal transduction pathway appear to be defective in genes that encode transcription factors. PIF3 was found not only to interact directly with PhyB but also with the promoters of many light-regulated genes (Ni et al., 1999; Martinez-Garcia et al., 2000). The hfr1/rsf1/rep1 mutants, on the other hand, appeared to be specific for PhyA pathway (Fairchild et al....

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