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The photo above shows the effects of the R allele of the pea shape gene on the synthesis of an enzyme that converts unbranched starch into branched starch. The r allele of this gene determines an inactive form of the enzyme, leading to a buildup of linear, unbranched starch that ultimately causes seed wrinkling. My question is, How does the r allele specifically prohibit the activity of the enzyme?

  • Does the DNA/Gene sequence simply code for proteins that create an enzyme which cannot bind to its respective substrate ?
  • Is the substrate in the above illustration the unbranched starch itself ?
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Bhattacharyya M, Martin C, Smith A. 1993. The importance of starch biosynthesis in the wrinkled seed shape character of peas studied by Mendel. Plant Mol Bio 22(3):525-531

Abstract:

The wrinkled-seed mutant (rr) of pea (Pisum sativum L.) arose through mutation of the gene encoding starch-branching enzyme isoform I (SBE1) by insertion of a transposon-like element into the coding sequence. Two isoforms of starch-branching enzyme have been documented in the developing pea embryo. The second isoform, SBEII, is expressed towards the later stages of embryo development while SBEI is expressed highly in the early stages. Due to mutation of SBEI the total amount of starch and the proportion of amylopectin, a branched starch polymer, are greatly reduced in the wrinkled (rr) line as compared to that in the wild-type, round (RR) line. Consequently, the level of sucrose in the rr line is nearly two fold that of the RR line. Increased sucrose concentration in the developing embryos of this mutant line causes increased uptake of water and thereby increases the cell size and fresh weight. During seed maturation in these mutant seeds a greater loss of water occurs. As a result, the wrinkled seed phenotype develops. Besides this morphological variation, the mutation also causes changes in the amount of lipid and of one storage protein, legumin. This review article discusses the role of the SBEI enzyme in causing such metabolic changes in the developing embryos with the implication that metabolism can play a central role in plant development.

These authors claim that the mutant protein is not present in peas, so the wrinkled phenotype is not due to an inability of the protein to bind its substrate (amylose and amylopectin - the linear and branched components of starch, respectively) but rather its complete absence. The mutant is due to an 800 bp transposon insertion into the coding sequence of SBE1. If it was expressed, it would be reasonable to expect that it wouldn't be able to bind its substrate. The authors also discuss mutations at other loci that have been shown to be responsible for the wrinkled phenotype, but it is believed that these are relatively newer and would not have been observed by Mendel.

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