Analysis of Sequence Microheterogeneity Among Zein Messenger RNAs

We have synthesized cDNA clones for maize zein proteins using mRNAs purified from developing endosperm. Analysis of these clones by in vitro translation of hybrid-selected mRNAs suggested differences in sequence homology among the mRNAs for the different molecular weight zein polypeptides. These differences were also apparent in restriction maps of clones corresponding to the Mr = 22,000, 19,000, and 15,000 zeins. Using radioactive cDNA inserts as probes, we measured the extent of sequence homology among zein clones with a sensitive dot hybridization procedure. By this analysis, it was possible to distinguish clones corresponding to the different molecular weight zeins at low (Tm - 49 degrees C) to moderate (Tm - 35 degrees C) criteria, while under more stringent conditions (Tm - 20 degrees C), distinctions could be made between zein sequences within a molecular weight group. This analysis distinguish three different mRNAs for each of the Mr = 22,000 and Mr = 19,000 zeins, but only one was detected for the Mr = 15,000 zein. Comparison of the nucleotide sequences of clones for the Mr = 22,000 and Mr = 19,000 zeins showed about 60% homology throughout the coding regions. This analysis also revealed the presence of short repetitive nucleotide sequences corresponding to tandem repeats of approximately 20 amino acids in both groups of proteins.