Zein storage protein gene family of Maize: An assessment of heterogeneity with cloned messenger RNA sequences

Zein messenger RNAs are shown to have an average poly(A) tract of 110 residues and a G + C content of 50%. Duplex DNA was enzymatically synthesized using purified zein m RNAs as the template material and inserted into a bacterial plasmid vector. Clones were rapidly screened for the size of the DNA insert by direct extraction of DNA from single colonies. A set of 20 clones with varying sizes was selected for further characterization. The 18 recombinant clones that contained zein sequences hybridized specifically to mRNA encoding polypeptides in either the light or the heavy zein class. The clones were further subgrouped by the presence of similar internal restriction sites and by the relative degree of sequence homology. The clones could be divided into two heavy class subgroups and at least three light class subgroups. In hybridizations of Southern transfers, representative clones from each of four subgroups reacted with numerous restriction fragments of genomic DNA. While each probe appeared to recognize common genomic sequences at the stringency employed, they each also hybridized to other bands. These results substantiate the contention that zein is a multigene family composed of sequences with limited heterogeneity. Although zein is being synthesized in large amounts in the endosperm at mid-development, no rearrangements or amplification of genomic endosperm sequences seem to be implicated.