Figure 3.

ZPR Family of Genes.

(A) Placement of leucine zipper (blue) in each of the four Arabidopsis ZPR proteins.

(B) Alignment of leucine zippers from the Arabidopsis REV (class III HD-ZIP), ZPR1 (At2g45450), ZPR2 (At3g60890), ZPR3 (At3g52770), ZPR4 (At2g36307), HD-ZIP1 (class I HD-ZIP protein), HD-ZIP2 (class II HD-ZIP), and GL2 (class IV HD-ZIP) proteins. REV and ZPR proteins have leucine zippers that are six heptads in length. The class IV HD-ZIP proteins have an insertion in the leucine zipper relative to the class III HD-ZIP proteins.

(C) Helical wheel model for interactions between the leucine zipper in the ZPR proteins and the REV protein. Proteins would be arranged in a parallel conformation. h1, h2, etc., indicate heptad 1, heptad 2, etc.

(D) Phylogenetic tree of all four Arabidopsis, all five rice, and two maize ZPR proteins. Alignment of the ZPR proteins was performed using the ClustalW algorithm. Bayesian phylogenetic analysis was conducted on the aligned protein data set using MrBayes version 3.1.2 (Huelsenbeck and Ronquist, 2001). Default settings were used, and the program was allowed to run for 100,000 generations. Trees were summarized after discarding the first 25,000 generations. The number above each branch corresponds to the posterior probability for that node.

(E) Model for a negative feedback loop between HD-ZIPIII proteins and ZPR proteins. Active HD-ZIPIII dimers induce the transcription of ZPR genes. The ZPR proteins dimerize with the HD-ZIPIII proteins, preventing them from forming homodimers and thus inactivating them. (In this model, we do not distinguish between active HD-ZIPIII dimers that are true homodimers, e.g., REV homodimers, or HD-ZIPIII dimers that consist of two types of HD-ZIPIII proteins, e.g., REV/PHB dimers. The extent to which mixed HD-ZIPIII dimers are important is not currently known.)