Quantitative analysis of the accumulation of Zein mRNA during maize endosperm development

In order to characterize the heterogeneity and expression of maize zein genes, we constructed and characterized a cDNA library of endosperm mRNAs. Clones from the library that were of sufficient size to be full-length or near full-length copies of zein mRNA were characterized by restriction enzyme mapping and cross-hybridization analysis. Based on these comparisons we found three classes of zein sequences corresponding to proteins of Mr 22,000, five corresponding to proteins of Mr 19,000, and a single one corresponding to a protein of Mr 15,000. Representative clones from these nine groups were used as probes to measure levels of the corresponding mRNAs in developing endosperms. It was found that these groups represent varying amounts of transcripts that range from 2 to 20% of the total endosperm mRNA population. For the Mr 19,000 and Mr 22,000 zein clones there is a correlation between the amount of mRNAs and the apparent number of genes in the genome. The relative level of mRNA for the Mr 15,000 zein was found to be 3 times that of the Mr 22,000 and Mr 19,000 zeins, suggesting that these genes are transcribed at a higher rate during endosperm development or that their mRNAs are more stable.