Expression of Storage Protein Multigene Families in Developing Rice Endosperm

Rice seeds accumulate both glutelins and prolamines as storage proteins with the former polypeptides being the predominant reserve. To evaluate the expression of the genes that encode for these proteins during seed development, the transcription activities and mRNA levels of several gene classes that comprise these multigene families were assessed. Results from nuclear run-on transcription assays and RNA blot studies showed that the expression of these gene classes for each multigene family was not coordinate but instead differentially regulated at both the transcriptional and post-transcriptional levels. Quantification of their total mRNA levels revealed equimolar amounts of their mRNAs in 5 and 10 day old seeds and a 40% excess of prolamine transcripts, relative to glutelin transcripts, in seeds of 15 days and older. In spite of the significant steady state levels of prolamine transcripts, glutelin mRNAs were detected at two-fold greater excess in membrane bound polysomes throughout seed development, thereby accounting for, at least in part, the greater accumulation of this storage protein in rice endosperm. These results suggest that, in addition to regulation at the transcriptional/post-transcriptional levels, ER membrane-associated translational control is also involved in the expression of rice seed storage protein multigene families.