Gene structure, protein structure, and regulation of the synthesis of a sulfur-rich protein in pea seeds

Two low molecular weight pea seed albumins (Mr approximately 6000 and approximately 4000) have been characterized by protein, cDNA, and gene sequencing. Both proteins are encoded by separate regions of the same mRNA species. The initial translation product is a preproprotein from which a signal sequence is removed co-translationally. The resultant proprotein (PA1) is then cleaved post-translationally to yield the mature form of the two albumins (PA1a and PA1b). Comparison of cDNA and protein sequences suggests that at least four different PA1 genes are expressed in the pea genome. Both PA1a and PA1b have an unusually high cysteine content (7.5 and 16.2%, respectively). Pea seeds developing under suboptimal levels of sulfur nutrient supply contain reduced levels of PA1 mRNA and accumulate greatly reduced levels of PA1a and PA1b in the mature seed. In vitro transcription studies showed that this reduced level of PA1 mRNA resulted from reduced post-transcriptional stability rather than an altered rate of transcription of the PA1 gene. In contrast, during normal seed development, the level of PA1 mRNA seems to be under transcriptional control. Sequence comparisons reveal some homology between PA1 and a number of low molecular weight proteins from seeds of a wide range of mono- and dicotyledonous plants.