Tissue-specific effects of maize bronze gene promoter mutations induced by Ds1 insertion and excision

Bz-wm is an allele of the Bz locus of maize isolated by McClintock (1962) as a derivative of bz-m2. It contains a Ds1 insertion 63 bp upstream of the start of transcription and a 3 bp insertion in the coding region at the site of the Ac element that was present in bz-m2. Bz-wm produces, in the aleurone layer of the endosperm, low amounts (approximately 1% of wild-type) of a Bz-gene encoded UDP-glucose: flavoid 3-0-glucosyltransferase (UFGT) polypeptide with altered thermal stability. Three phenotypically wild-type derivatives, Bz' (wm)-1, Bz' (wm)-2 and Bz' (wm)-3, were isolated in the presence of Ac and shown to have excised the Ds1 element but not fully restored UFGT activity in endosperm assays. In the studies reported here, we have further analyzed these Bz' derivatives of Bz-wm by determining the DNA sequences left behind on Ds1 excision, and by measuring the amount of UFGT activity and/or Bz mRNA conditioned by Bz-wm and the Bz' derivatives in different tissues. The data indicate that tissue-specific differences in expression of the Bz gene have been produced in alleles with mutations caused by transposable elements Ac and Ds. These mutations may affect either the amount of Bz transcription or the stability of the UFGT polypeptide. The sequence or spacing in the -63 region of the Bz promoter appears to be critical for maximum expression in aleurone and husk but not in pollen and pigmented seedling tissue.