Maize genes encoding the small subunit of ADP-glucose pyrophosphorylase

Plant ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme comp osed of two large and two small subunits. Here, we report the structures of the maize (Zea mays) genes encoding AGP small subunits of leaf and endospe rm. Excluding exon 1, protein-encoding sequences of the two genes are nearl y identical. Exon I coding sequences, however, possess no similarity. Intro ns are placed in identical positions and exhibit obvious sequence similarit y. Size differences are primarily due to insertions and duplications, hallm arks of transposable element visitation. Comparison of the maize genes with other plant AGP small subunit genes leads to a number of noteworthy infere nces concerning the evolution of these genes. The small subunit gene can be divided into two modules. One module, encompassing all coding information except that derived from exon 1, displays striking similarity among all gen es. It is surprising that members from eudicots form one group, whereas tho se from cereals form a second group. This implies that the duplications giv ing rise to family members occurred at least twice and after the separation of eudicots and monocot cereals. One intron within this module may have ha d a transposon origin. A different evolutionary history is suggested for ex on 1. These sequences define three distinct groups, two of which come from cereal seeds. This distinction likely has functional significance because c ereal endosperm AGPs are cytosolic, whereas all other forms appear to be pl astid localized. Finally, whereas barley (Hordeum vulgare) reportedly emplo ys only one gene to encode the small subunit of the seed and leaf, maize ut ilizes the two genes described here.