The genomic features of the small subunit ADP-glucose pyrophosphorylase (AGPase) isoforms are different in barley and maize. The two isoforms found in barley originated from one single gene through alternative splicing, while two independent genes encode the two isoforms in maize. To ascertain the genomic organizations of two small subunit AGPase isoforms in sweetpotato (ibAGP1 and ibAGP2), we isolated genomic DNAs containing the entire coding regions of two genes. Complete genomic structures of ibAGP1 and ibAGP2 were ascertained by the sequencing of these genomic regions. The transcribed regions of ibAGP1 and ibAGP2, comprising nine exons and eight introns, were distributed over 3.9 and 4.0 kb, respectively. The eight introns differed in length, from 76 to 946 bp in ibAGP1, and from 76 to 811 bp in ibAGP2, while the locations of introns in ibAGP1 and ibAGP2 were identical. There was 46-58% sequence identity between the intron sequences of the two genes. Intron sequence analyses suggested that either duplication in each intron, or gene conversion between introns of two isoforms, might cause major intron size differences between the two genes. Altogether, these results indicate that two small subunit AGPase isoforms in sweetpotato are encoded by two independent genes, in a fashion similar to that of maize small subunit AGPase genes.
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