Evolutionary pattern of angiosperm bZIP factors homologous to the maize Opaque2 regulatory protein

Opaque2 (02) is a bZIP transcriptional regulatory factor involved in the control of seed storage proteins synthesis as well as carbon and nitrogen metabolism during maize seed development. Phylogenetic analysis of a possible complete and nonredundant collection of angiosperm bZIP factors resulted in the identification of 20 angiosperm O2-homologues that defined what we call the 02 gene family. Members of the family share a highly conserved bZIP DNA binding domain and several other motifs which define important functional features. The 02 family was enriched by the identification of 25 new putative angiosperm 02 homologous genes in EST databases and in the rice genome. Based on parsimony analysis, the collection of 02 homologues was organized into one eudicot-monocot and three monocot groups of orthologous genes and two groups of eudicot genes. These results support a model of the evolution of the 02 family that involves two 02 homologous gene duplications before the separation of monocots and eudicots. Further expansion of 02 homologues resulted in at least three and one gene duplications in the monocot and eudicot lineages, respectively. 02 appears to have been the result of a monocot-specific gene duplication event, and the possibility that 02 represents a functional specialization restricted to monocots is suggested.