Comparison of the exon-intron structures of PRR genes around the region of PR-domain and CCT-motif. The amino acid sequences encoded by PRR genes were aligned using TCoffee program . Accession numbers and gene IDs of the PRR genes are shown in Additional file 1. The numerals on the right side indicate the numbers of amino acid residues of each protein. Sequence similarity is indicated below the alignment using the symbols "asterisk," "colon," and "dot" for identical, highly similar, and weakly similar residues, respectively. Black and gray shadings on the alignments indicate a site of exon-intron boundary and one-amino acid deletion, respectively.