THE HOMEOBOX GENE ATH1 OF ARABIDOPSIS IS DEREPRESSED IN THE PHOTOMORPHOGENIC MUTANTS COP1 AND DET1

A light-regulated Arabidopsis thaliana homeobox 1 gene (ATH1) was iden tified in a transcription factor gene collection. Primer extension ana lysis of this gene showed the presence of two major transcripts with u nusually long 5' untranslated leader sequences. The leader sequence of the isolated cDNA clone contains several small open reading frames up stream of the initiation codon of the largest open reading frame codin g for the homeodomain protein. This leader sequence was found to affec t the translation efficiency negatively in an in vitro translation sys tem. The expression of the ATH1 gene is dependent on the presence of l ight. After illuminating etiolated or dark-adapted Arabidopsis seedlin gs, the ATH1 mRNA level increased rapidly. Expression of ATH1 does not require the presence of active chloroplasts because photooxidative de struction of the chloroplast by norflurazon treatment did not influenc e the ATH1 mRNA level. In dark-grown seedlings of the photomorphogenic constitutive photomorphogenic 1 (cop1) and deetiolated 1 (det1) mutan ts, the ATH1 mRNA level was elevated. This shows that the gene product s of these loci directly or indirectly repress ATH1 expression in etio lated wild-type seedlings. A correlation between the strength of the c op1 allele and the ATH1 mRNA level was found. This relationship sugges ts a role for the ATH1 protein in the signal transduction pathway down stream of COP1.