Portal de Periódicos da CAPES

The missing step of the l -galactose pathway of ascorbate biosynthesis in plants, an l -galactose guanyltransferase, increases leaf ascorbate content

2007; National Academy of Sciences; Volume: 104; Issue: 22 Linguagem: Inglês

10.1073/pnas.0701625104

1091-6490

William A. Laing, Michele Wright, Janine M. Cooney, Sean Bulley,

Photosynthetic Processes and Mechanisms

The gene for one postulated enzyme that converts GDP- l -galactose to l -galactose-1-phosphate is unknown in the l -galactose pathway of ascorbic acid biosynthesis and a possible candidate identified through map-based cloning is the uncharacterized gene At4g26850. We identified a putative function for At4g26850 using PSI-Blast and motif searching to show it was a member of the histidine triad superfamily, which includes d -galactose uridyltransferase. We cloned and expressed this Arabidopsis gene and the homologous gene from Actinidia chinensis in Escherichia coli and assayed the expressed protein for activities related to converting GDP- l -galactose to l -galactose-1-P. The expressed protein is best described as a GDP- l -galactose-hexose-1-phosphate guanyltransferase (EC 2.7.7.), catalyzing the transfer of GMP from GDP- l -galactose to a hexose-1-P, most likely d -mannose-1-phosphate in vivo . Transient expression of this A. chinensis gene in tobacco leaves resulted in a >3-fold increase in leaf ascorbate as well as a 50-fold increase in GDP- l -galactose- d -mannose-1-phosphate guanyltransferase activity.