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 identified in a transcription factor gene collection. Primer extension analysis of this gene showed the presence of two major transcripts with unusually long 5' untranslated leader sequences. The leader sequence of the isolated cDNA clone contains several small open reading frames upstream of the initiation codon of the largest open reading frame coding for the homeodomain protein. This leader sequence was found to affect the translation efficiency negatively in an in vitro translation system. The expression of the ATH1 gene is dependent on the presence of light. After illuminating etiolated or dark-adapted Arabidopsis seedlings, the ATH1 mRNA level increased rapidly. Expression of ATH1 does not require the presence of active chloroplasts because photooxidative destruction of the chloroplast by norflurazon treatment did not influence the ATH1 mRNA level. In dark-grown seedlings of the photomorphogenic constitutive photomorphogenic 1 (cop1) and deetiolated 1 (det1) mutants, the ATH1 mRNA level was elevated. This shows that the gene products of these loci directly or indirectly repress ATH1 expression in etiolated wild-type seedlings. A correlation between the strength of the cop1 allele and the ATH1 mRNA level was found. This relationship suggests a role for the ATH1 protein in the signal transduction pathway downstream of COP1.