The plastidic phosphoglucomutase from Arabidopsis. A reversible enzyme reaction with an important role in metabolic control

An Arabidopsis cDNA (AtPGMp) encoding the plastidic phosphoglucomutase (PGM ) predicted a 623-amino acid protein with an N-terminal sequence typical of a plastid signal peptide. Expression of a recombinant protein in Escherich ia coli confirmed its enzyme activity. The recombinant enzyme had an appare nt K-m value of 98.5 ELM and a V-max of 4.48 mu mol min(-1) (mg protein)(-1 ). The Calvin cycle intermediates fructose-1,6-bisphosphate and ribulose-1, 5-bisphosphate exerted an inhibitory effect on PCM activity, supporting its proposed involvement in controlling photosynthetic carbon flow. A point mu tation was identified in the AtPGMp gene of the Arabidopsis pgm-1 mutant. T he mutation in the mutant transcript generated a stop codon at about one th ird of the wild-type open reading frame, and thus rendered the polypeptide nonfunctional. Storage lipid analysis of the pgm-l mutant seeds showed a 40 % reduction in oil content compared with that of wild type. Our results ind icate that plastidic PGM is an important factor affecting carbon flux in tr iacylglycerol accumulation in oilseed plants, most likely through its essen tial role in starch synthesis.