Expression of the P-wr allele of the P gene
--Surinder Chopra and Thomas Peterson

The P gene encodes a Myb-homologous transcriptional regulator of the flavonoid biosynthetic genes in the pathway leading to the production of a red flavonoid pigment called phlobaphene. Alleles of the P gene differ strikingly with regard to the distribution of phlobaphenes in kernel pericarps and cob glumes. We have previously characterized the P-rr allele, which specifies red pericarps and red cob glumes, and we are now investigating the mechanism of organ-specific expression of the P-wr allele, which specifies colorless pericarps and red cob glumes. RNA blot and RT-PCR analyses have shown that both colorless pericarps and the red cob glumes of P-wr plants contain the P-wr-specific transcripts. However, transcripts of the genes C2, Chi and A1, encoding enzymes for flavonoid biosynthesis, are present at a very low level in P-wr pericarps between 11 to 22 DAP (days after pollination). Using PCR methods, we have amplified P-wr-specific cDNAs from pericarps and cob glumes. Compared to P-rr, the P-wr 3' end and thus the -COOH terminus of the protein are completely different. In contrast, the 5' regions of P-rr and P-wr are more similar, but contain several changes in the Myb-homologous DNA binding domain. One of the sequence differences in the P-wr protein would substitute a tyrosine residue for a conserved cysteine in helix 3 of tryptophan repeat 1 in the Myb-homologous domain; this change will be investigated as a possible cause of tissue-specific activity of the P-wr protein.
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