Putative gene homologues of trithorax in Arabidopsis.

摘要

Our major source of information on the mechanisms involved in processes regulating cell fate and developmental programs has been acquired from studies in animals. Plant homologs of the trithorax genes, as antagonists of Polycomb group genes, have not been characterized yet. We have isolated two Arabidopsis genes (ATX1 and ATX2), as first examples of plant genes of the Trithorax family. They are highly similar but display different tissue and development expression patterns. ATX1 was ubiquitously expressed with highest levels registered in young seedlings. ATX2 was less active in all tested tissues, was not expressed in mature leaves but was highly expressed in roots. Despite the high level of homology between them, their expression pattern in the various tissues tested and at different developmental stages suggested different functions for the two genes. ATX1 positively regulates the flower homeotic genes APETALA1, APETALA2, PISTILLATA and AGAMOUS, and is responsible for maintaining the levels of their mRNAs in immature flowers. Mutant atx1 Arabidopsis plants have impaired growth and homeotic floral alterations, suggesting developmental functions for the gene. ATX1 is defined as an activator of flower homeotic genes. The effects of ATX1 are highly specific. Like the animal counterpart, ATX1 does not appear to be involved in establishing an active state for the homeotic genes, but rather in maintaining it. This is the first evidence of the pleiotropic function of an Arabidopsis homolog of trithorax. We have defined a new structural domain in the molecular structure of ATX1 and ATX2. The possible function of this motif (DAST) has not been elucidated yet but, interestingly, DAST was found in genes of animal and plant origin only and is not present in bacteria and in yeast. We have systematized and comprehensively analyzed the currently available genes of three families, SU(VAR)3-9, E(Z), and TRITHORAX. This whole-genome comparative approach outlined an informative picture of a significant heterogeneity inside each family, unexpected relationships among same-family members and a tight correlation between the level of amino acid homology of the SET domain and the evolution of the entire architecture of any given gene.