UV-responsive genes of Arabidopsis revealed by similarity to the Gcn4-mediated UV response in yeast

A UV response that involves the Ras proteins and AP-1 transcription factors has recently been described in mammals and yeast. To test whether an equiv alent response exists in plants, we monitored the expression of Arabidopsis histidinol dehydrogenase gene (HDH), a homologue of the yeast HIS4 gene, w hich is strongly induced by UV light and is a target of the transcriptional activator Gcn4. We show that HDH mRNA levels increase specifically in resp onse to UV-B light. Only small increases were detected upon exposure to oth er wavelengths. To isolate plant genes involved in this UV response, a gcn4 mutant was transfected with an Arabidopsis thaliana cDNA library. A new ty pe of nucleotide diphosphate kinase (NDPK Ia) with a significant homology t o the human tumor suppressor protein Nm23 rescued the gcn4 phenotype. NDPK Ia specifically binds to the HIS4 promoter in vitro and induces HIS4 transc ription in yeast. In Arabidopsis, the NDPK Ia protein is located in the nuc leus and cytosol, Expression studies in seedlings revealed that the level o f NDPK la mRNA, like that of HDH, increases in response to UV-B light. It a ppears that NDPK Ia and HDH are components of a novel W-responsive pathway in A. thaliana.