Expression of holo and apo forms of spinach acyl carrier protein-I in leaves of transgenic tobacco plants.

Acyl carrier protein (ACP) is a chloroplast-localized cofactor of fatty acid synthesis, desaturation, and acyl transfer. We have transformed tobacco with a chimeric gene consisting of the tobacco ribulose-1,5-bisphosphate carboxylase promoter and transit peptide and the sequence encoding the mature spinach ACP-I. Spinach ACP-I was expressed in the transformed plants at levels twofold to threefold higher than the endogenous tobacco ACPs as determined by protein immunoblots and assays of ACP in leaf extracts. In addition to these elevated levels of the holo form, there were high levels of apoACP-I, a form lacking the 4'-phosphopantetheine prosthetic group and not previously detected in vivo. The mature forms of both apoACP-I and holoACP-I were located in the chloroplasts, indicating that the transit peptide was cleaved and that attachment of the prosthetic group was not required for uptake into the plastid. There were also significant levels of spinach acyl-ACP-I, demonstrating that spinach ACP-I participated in tobacco fatty acid metabolism. Lipid analyses of the transformed plants indicated that the increased ACP levels caused no significant alterations in leaf lipid biosynthesis.