Regulation of gene expression in developing pea seeds
Three classes of legumin, encoded by the gene sub-families legA, legJ and legS, and a lectin encoded by a single gene, lecA, accumulate in the developing cotyledons of Pisum sativum L. Transcription rates for the genes encoding these proteins were measured in nuclei isolated from cotyledons at 12 and 16 days after flowering (DAF). The steady-state levels of the corresponding mRNA species were also measured, in absolute terms, throughout cotyledon development. It was found that the different legumin gene sub-families are not coordinately expressed and, in addition, members within the legJ sub-family show differential temporal expression. Also, it was demonstrated that the length of the poly (A) tail of the lectin mRNA is reduced during the period when the steady-state level of this mRNA is in decline. When transcription rates and steady-state mRNA levels of the different gene families are compared, there is little correlation. This suggests a posttranscriptional regulation of the quantitative level of expression of these genes. Expression of the legumin genes is known to be seed-specific, whereas expression of the lectin gene occurs in the root as well as the seed. When transcription rates were measured in leaf nuclei the levels of legumin and lectin transcripts detected approached background levels, indicating that these genes are either inactive or transcribed at very low levels in leaf; however, the rate of transcription of the chlorophyll a/b binding-protein gene was high. This suggests transcriptional control as the major factor in the organ-specificity of legumin and lectin expression. The apparent posttranscriptional regulation of the quantitative level of expression of different seed-protein genes was investigated further by pulse-chase labelling the RNA of pea cotyledons grown in culture. Also, the possibility of using cell-free extracts to assay the cytoplasmic stability of specific polysomal mRNAs was investigated.