Transcriptional regulation of the maize Myb-homologous genes p1 and p2, and molecular analysis of a novel variegated pericarp allele
In maize, the p1 gene encodes a Myb-like factor that activates transcription of structural genes required for red phlobaphene pigmentation in certain floral organs including silk, kernel pericarp, and cob. A second maize gene, p2, is homologous to p1 and shares a similar function in transcriptional activation of flavonoid biosynthetic genes and co-regulation of maysin level in silk. The p2 gene is expressed in silk and anther, but not in pericarp and cob. Sequence comparison of p1 and p2 genes revealed that they share a 90 bp sequence in the promoter immediately upstream of the transcription start site. In chapter 2 of this thesis, we report the molecular analysis of a new p1 mutant allele p1-vv85. The p1-vv85 allele exhibits high somatic instability and frequent germinal reversion; however, no structural differences were detected in the p1-locus between p1-vv85 and its revertant allele P1-rr'85. A novel CACTA element was isolated from the second intron of the p1 gene in both p1-vv85 and P1-rr'85. Transcriptional analysis indicated that transcription of the p1 gene in p1-vv85 initiates at a normal rate as in P1-rr'85, but elongation terminated prematurely in the vicinity of the CACTA insertion element. Interestingly, sequences in the CACTA element TIRs were methylated in P1-rr'85, but not in p1-vv85. We propose that reversion of p1-vv85 to a functional P1-rr state results from relief of suppression caused by binding of a trans-acting factor. In chapter 3, we report the results of functional analysis of the maize p2 gene promoter using both transient and stable transformation assays. A series of p2-promoter::GUS constructs were introduced into maize cells by particle bombardment. The results suggest that the p2 promoter sequences required for expression in cultured cells and silk are relatively small. The TATA box region in the promoter is important for promoter activity. Additionally, some potential cis-elements of the p2 promoter respond differently in cultured cells and silks. In summary, transcriptional analyses of the p1-vv85 allele and the p2 gene have provided a deeper understanding of the effects of transposable elements on gene expression, and the tissue-specific regulation of plant genes.