Two Paralogous Genes Encoding Small Subunits of ADP-glucose Pyrophosphorylase in Maize, <Emphasis Type="Italic">Bt2</Emphasis> and <Emphasis Type="Italic">L2</Emphasis>, Replace the Single Alternatively Spliced Gene Found in Other Cereal Species |
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Authors: | Sandrine Rösti Kay Denyer |
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Institution: | (1) John Innes Centre, Norwich Research Park, Norfolk, NR4 7UH, UK |
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Abstract: | Two types of gene encoding small subunits (SSU) of ADP-glucose pyrophosphorylase, a starch-biosynthetic enzyme, have been
found in cereals and other grasses. One of these genes encodes two SSU proteins. These are targeted to different subcellular
compartments and expressed in different organs of the plant: the endosperm cytosol and the leaf plastids. The SSU gene encoding
two proteins evolved from an ancestral gene encoding a single protein by the acquisition of an alternative first exon. Prior
to the work reported here, this type of SSU gene had been found in all grasses examined except maize. In maize, two separate
genes, Bt2 and L2, were known to have the same roles as the alternatively spliced gene found in other grasses. The evolutionary origin of these
maize genes and their relationship to the SSU genes in other grasses were unclear. Here we show that Bt2 and L2 are paralogous genes that arose as a result of the tetraploidization of the maize genome. Both genes derive from an ancestral
alternatively spliced SSU gene orthologous to that found in other grasses. Following duplication, the Bt2 and L2 genes diverged in function. Each took a different one of the two functions of the ancestral gene. Now Bt2 encodes the endosperm cytosolic SSU but does not contribute significantly to leaf AGPase activity. Similarly, L2 has lost
the use of one of its two alternative first exons. It can no longer contribute to the endosperm cytosolic SSU but is probably
responsible for the bulk of the leaf AGPase SSU.
Reviewing Editor: Dr. Patrick Keeling
Sequences have been deposited in GenBank under accession numbers AY727927, DQ118037, and DQ118038. |
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Keywords: | Starch Maize ADP-glucose pyrophosphorylase |
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