Presence of multiple cDNAs encoding an isoform of ADP-glucose pyrophosphorylase large subunit from sweet potato and characterization of expression levels

Three cDNAs (iAGPLI-1, iAGPLI-2, and iAGPLI-3) encoding an isoform of AGPas e large subunit were isolated from a sweet potato cDNA library constructed from tuberous root tissue. iAGPLI-1 was 2,161 bp in length and contained an open reading frame of 517 amino acids with a calculated molecular mass of 57,689Da, iAGPLI-2 and iACPLI-3 were 1,804 and 1,524 bp in length, respecti vely, and contained partial open reading frames of 490 and 385 amino acids. Deduced amino acid sequence comparison analysis showed that iAGPLI-1 has s equence identity with MGPtl-t (97.9) and iAGPLI-3 (98.7%) while iAGPLI-2 an d iACPLI-3 have 96.8% sequence identity. 1AGPLI-1 had the highest sequence identity of 77.8% with potato ACPase (sAGPL1). Steady-state levels of iAGPL I-1 transcripts were expressed predominantly in the stem, and moderately in the tuberous root, but not in either the roots or leaves, However, AGPase activity was present in all tissues, The erpression level in the stem decli ned dramatically after a 12 h incubation in the dark to nearly 3% of the va lue under light, although the activity under a dark condition remained at h alf the levels in light, The activity levels were not correlated with the t ranscript levels, iAGPL transcripts in leaves were induced by sucrose feedi ng but not by glucose or fructose, Therefore, the expression of iAGPLI-1 is regulated in stem tissue preferentially and by sucrose, Southern blot anal ysis showed that the sweet potato genome contained several copies of iACPLI gene probably due to polyploidy.