Complex structure of a maize Myb gene promoter: functional analysis in transgenic plants

The maize P gene encodes a Myb-like transcription factor that regulates syn thesis of red flavonoid pigments in floral organs. To study the transcripti onal regulation of the P gene, candidate regulatory sequences of the P1-rr gene promoter were identified by Ac insertional mutagenesis and subjected t o functional testing in transgenic maize plants. The results indicate that a 561 bp fragment (Pb) encompassing the transcription start site (-235 to 326) supports weak expression of a GUS reporter gene in floral organs, incl uding husk, silk, kernel pericarp, cob and male inflorescence. Two other fr agments, located approximately 1 and 5 kb 5' of the transcription start sit e, increased the levels of GUS activity in floral tissues and thus appear t o contain enhancer elements. All of the tested constructs gave similar patt erns of GUS expression, suggesting that the 561 bp Pb fragment that is comm on among the transgene constructs contains regulatory elements that promote activation in floral organs. The basal promoter and proximal enhancer frag ments contain putative binding sites for bZip regulatory factors, and a com plex arrangement of palindromes including a large inverted repeat of two tR NA-like genes. Possibly, interconversions between linear and cruciform conf ormations of the palindromes may affect protein/DNA interactions and thereb y modulate P1-rr expression.