| Acid pbosphatases (APases) catalyze the hydrolysis of orthophosphate esters at acid pH optima. APases have been detected in many plant tissues, including roots, tubers, leaves, and developing seedlings. APase induction is a major part of the plant phosphate-starvation response, and both cell-wall and secreted APases appear in the roots of phosphate-starved plants. Previous work identified an APase from potato tuber possessing unusually high activity. A genomic clone for potato tuber acid phosphatase (PTAP) was isolated, along with several other clones showing amino acid sequence homology to acid phosphatases. One of these, pap7, was sequenced and characterized in this study. Of the 18 kb clone, 6.6 kb of contiguous sequence was assembled, containing APase-homologous sequence. The sequence comprised seven predicted exons, with all the boundaries but one conforming to plant splice site consensus sequences. The predicted protein displayed 53% identity with PTAP, 63% identity with red kidney bean APase, and 61% identity with sweet potato APase. The amino acids believed to be critical for active site binding and catalysis in kidney bean and sweet potato APases were completely conserved in both PAP7 and PTAP. Southern blotting of genomic DNA indicates one copy of pap7 is present in the potato genome. RNA from phosphate-fed and starved potato plants was probed with pap7 but no expression was found in leaves, roots or stems. Pap7 appears to be a member of a family of related plant acid phosphatase genes including the "purple" APases and a number of uncharacterized genes from diverse species. Although it does not appear to be phosphate-starvation induced, pap7 could be expressed under other APase-inducing conditions, including senescence, salt stress, flowering, or seed germination. |
