Structure and expression of the potato ADP-glucose pyrophosphorylase small subunit

ADP-glucose pyrophosphorylase (AGP) catalyzes a key regulatory step in starch synthesis. To elucidate the molecular basis for the expression of the potato (Solanum tuberosum L.) AGP during tuber development, the structure of the small subunit AGP (sAGP) gene and its patterns of expression were examined. DNA sequence analysis revealed that the sAGP gene is over 5.5 kilobases long and has a complex structure including eight introns. Unlike the situation in other plants where tissue-specific sAGP are found, our Southern and Northern blot analysis indicated that the same sAGP gene is expressed both in tubers (non-photosynthetic tissue) and leaves (photosynthetic tissue). These data were supported by comparing sequences of isolated sAGP leaf cDNAs to the tuber cDNA sequence, by primer extension analysis of leaf and tuber poly(A)+ RNAs, and by the spatial expression patterns of a gusA (beta-glucuronidase) reporter gene driven by the potato sAGP promoter in transgenic potato plants. Although the sAGP gene appeared to be transcriptionally controlled in both developing tubers and in leaves, the relative level of leaf antigen was significantly lower than its level of transcript, indicating that sAGP expression in leaves is primarily regulated post-transcriptionally. The observed tissue type-dependent regulation of sAGP expression appears to control the extent of starch biosynthesis by regulating the levels of this enzyme and, thus, alleviate the need for tissue-specific forms of the sAGP in potato.