Gene products and structure analysis of wild-type and mutant alleles at the Opaque-2 locus of Zea mays

The Opaque2 b-ZIP transcriptional activator is involved in the transcriptional regulation of genes coding for different types of proteins in the subaleurone layers of maize endosperm cells. Three wild-type (O2-w1, O2-w2 and O2-w3) and five mutant alleles (o2-T, o2-52, o2-R, o2-R and o2-676) were investigated. Western analyses with specific amino terminal and carboxy terminal antisera revealed the presence of differentially shortened o2 polypeptides for some o2 alleles, The nucleotide sequences of the wild-type alleles showed, for the deduced polypeptides, a highly conserved amino acid sequence having only a short deletion/insertion of a proline-glutamic acid motif and glycine residues within the amino terminal part of the protein. The o2-T allele is characterised by a 25 nucleotide deletion that introduces a stop codon 645 bases after the ATG start codon of the longest ORE This deletion produces a truncated polypeptide (o2-T) missing both the basic and the L-Zipper motifs. Similarly, the o2-52 allele contains an insertion of 4 base pairs 796 bases downstream from the start codon, which causes a frame shift, giving rise to another truncated polypeptide that lacks all the carboxy terminal part downstream from the second leucine of the L-Zipper. Conversely, the o2-It allele produces two polypeptides: one of them migrates at a slightly faster rate in SDS-PAGE than do the three wild-types and the second migrates to a position between those of the o2-T and o2-52 proteins. In fact, the o2-It nucleotide sequence shows a deletion of 10 base pairs 711 bases after the start codon, which causes a frame shift that gives rise to a premature stop codon 45 base pairs later. This deletion results in a product containing 252 aa residues.