STRUCTURAL FEATURES OF THE MAIZE SUS1 GENE AND PROTEIN

Genomic clones, cDNA clones, and protein of the maize (Zea mays L.) Su c synthase1 (sus1) gene were isolated and sequenced. Termini (5' and 3 ') of the transcribed unit were identified. The SUS1 protein was purif ied from tissue culture cells as a phosphorylated protein. The overall structure of sus1 is virtually identical with that of the paralogous gene, shrunken1 (sh1); however, the last intron of sh1 is missing in s us1. This intron bears much sequence similarity with the adjacent exon , suggesting that the intron arose from an internal duplication. Altho ugh the placement of the other 14 introns is identical in both genes, the introns exhibit markedly greater differences in size and sequence relative to that shown by the exons. An explanation for the differenti al rate of divergence of exons and introns is selection pressure for g ene function. Additionally, comparisons of coding regions of plant suc rose synthases show that sh1-like and sus1-like genes can be found in all monocots so far analyzed. These latter observations point to an im portant role played by both genes in this group of plants.