Alleles of the maize P gene with distinct tissue specificities encode Myb-homologous proteins with C-terminal replacements

The maize P gene is a transcriptional regulator of genes encoding enzymes for flavonoid biosynthesis in the pathway leading to the production of a red phlobaphene pigment. Multiple alleles of the P gene confer distinct patterns of pigmentation to specific floral organs, such as the kernel pericarp and cob tissues, To determine the basis of allele-specific pigmentation, we have characterized the gene products and transcript accumulation patterns of the P wr allele, which specifies colorless pericarps and red cob tissues. RNA transcripts of P-wr are present in colorless pericarps as well as in the colored cob tissues; however, the expression of P wr in pericarp does not induce the accumulation of transcripts from the C2 and Al genes, which encode enzymes for flavonoid pigment biosynthesis. The coding sequences of P-wr were compared with the P rr allele, which specifies red pericarp and red cob, The P wr and P rr cDNA sequences are very similar in their 5' regions. There are only two nucleotide changes that result in amino acid differences; both are outside of the Myb-homologous DNA binding domain. In contrast, the 3' coding region of P rr is replaced by a unique 210-bp sequence in P-wr. The predicted P-wr protein has a C-terminal sequence resembling a cysteine-containing metal binding domain that is not present in the P-rr protein. These results indicate that the differential pericarp pigmentation specified by the P rr and P-wr alleles does not result from an absence of P wr transcripts in pericarps. Rather, the allele-specific patterns of P rr and P wr pigmentation may be associated with structural differences in the proteins encoded by each allele.