Figure 5.

Model for the role of FIS protein products in seed development. The cartoon represent possible roles of FIS protein in maintaining Seed Development Gene (SDG) repression. Molecular events taking place in ovule and pollen are described. X and Y represent additional proteins that could be part of the FIS complex. (A) The “FIS complex” model. FIS1, 2, and 3 bind as a complex (FIS complex) to specific sites in SDG regulatory regions termed FRE for FIS Response Element. The FIS complex represses by interfering with the basal transcription machinery. (B) The “Sequential Regulatory” model. This model proposes that FIS2 transcriptionally regulates FIS1 and/or other FIS gene(s) and subsequently forms an active FIS complex, repressing SDG. In both models, FIS genes are not active in pollen so that SDG can be active at the appropriate stage of development.