THE HOMEOBOX GENE ATK1 OF ARABIDOPSIS-THALIANA IS EXPRESSED IN THE SHOOT APEX OF THE SEEDLING AND IN FLOWERS AND INFLORESCENCE STEMS OF MATURE PLANTS

The homeodomain is a DNA-binding domain present in a large family of e ukaryotic regulatory proteins. Homeodomain proteins have been shown to play key roles in controlling developmental programs in various organ isms. Here we report the isolation and characterisation of a homeobox gene from Arabidopsis thaliana designated ATK1. The gene was isolated using as a probe the homeobox domain of the KN1 gene from maize. The h omeodomain of ATK1 is highly homologous to the homeodomain of the KN1 gene of maize (81%) but shows only poor homology outside the homeodoma in. Therefore ATK1 is probably not the Arabidopsis homologue of the KN I gene from maize. It contains the four invariant amino acid residues present in the recognition helix 3 of all other homeodomain proteins. Outside the homeodomain a region rich in aspartate and glutamate resid ues is found suggesting that ATK1 is a transcriptional activator. The gene contains four introns which is similar in the KN1 gene of maize a nd the Osh1 gene of rice. Primer extension reveals the presence of two transcription initiation sites. The leader sequence of the genuine tr anscript is 342 nucleotides long and contains two upstream open readin g frames. ATK1 is strongly expressed in the shoot apex of seedlings, w hile in mature plants the gene is primarily expressed in flowers and i nflorescence stems. Such an expression pattern is reminiscent of that of the KN1 gene of maize and therefore ATK1 could similarly be involve d in determining cell fate.