Subfamily Organization and Phylogenetic Origin of Genes Encoding Plastid Lipid-Associated Proteins of the Fibrillin Type

Plastid lipid-associated proteins of the fibrillin type have been found in a number of plants, but beyond their structural role in carotenoid-storing fibrils in chromoplasts, their functions remain poorly understood. We have identified 13 genes encoding members of this family in Arabidopsis. Some genes can be grouped in subfamilies of closer sequence similarity. Fibrillin polypeptides share typical signa- tures and potential functional domains, but appear quite diverse when other parameters are considered, namely their molecular weight, pI, or hydrophobic profiles. Most of these genes show differential expres- sion when stress- or organ-related expression is examined. An evolutionary scheme for fibrillin-related genes can be proposed. Three Arabidopsis fibrillin genes originate from relatively recent duplications, and are not found in the rice genome, which has ten orthologues. Four genes are found in a red alga. A diverse situation is found in cyanobacteria. ! -cyanobacteria have one orthologue of one plant gene and, in some species, a second gene (without plant orthologue). " -Cyanobacteria have either one or two members of a third gene subfamily (without plant orthologue) or no such gene at all (in species adapted to low light). This suggests that plants multiplied their fibrillin genes from an ancestral gene that still has a relative in some cyanobacteria.