Two adjacent genomic zein sequences: structure, organization and tissue-specific restriction pattern

A clone isolated from a library of genomic clones from Zea mays contains two adjacent zein light-chain genes. The nucleotide sequence of the two coding regions and their flanking sequences shows that the two genes represent two different subclasses of light-chain zeins. One belongs to an abundant and highly conserved class and the other, which is 88% homologous to the first, is less represented in the genome of maize line W64A. Both genes present in clone zE are anomalous: one contains a mutation that introduces a termination codon after 39 amino acids, the other has the ATG initiation codon mutated to CTG. The presence of inversions, insertions and short inverted or direct repeats in the flanking sequences suggests that zein genes may be subject to rearrangements and transpositions. Southern blot analysis of genomic DNA indicates that the particular arrangement represented by clone zE is present at least four times in maize line W64A but other arrangements are found in this and other maize lines. DNA extracted from different tissues and developmental stages is digested differently by enzymes sensitive to methylation of their target sequence. The results indicate that zein genes contain methylated sites that are demethylated in correlation with their expression.