Poster: Secondary metabolism

Abs # 279: A pentose phosphate cycle shunt to isoprenoid biosynthesis (PPCI)*.

Presenter:	Poliquin, Kelly , poliquin@wam.umd.edu
Authors	Poliquin, Kelly  (A)   Cunningham, Jr., Francis X. (A)   Gantt, R. Raymond  (A)   Gantt, Elisabeth  (A)
Affiliations:	(A): University of Maryland, Department of Cell Biology and Molecular Genetics

In cyanobacteria many compounds, including chlorophylls, carotenoids, and hopanoids, are synthesized from the isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Isoprenoid biosynthesis is presumed to occur by the methylerythritol phosphate (MEP) pathway in Synechocystis PCC 6803. Stimulation of isoprenoid biosynthesis (from ¹⁴C-IPP in a cell-free extract) by pentose phosphate cycle (PPC) substrates and not by MEP substrates, and insensitivity to fosmidomycin, suggested other than a simple linear MEP pathway (Ershov et al. J. Bacteriol. 184:5045-5051). In exploring PPC stimulated isoprenoid synthesis, we sought to determine the role of LytB, the terminal enzyme in the MEP pathway leading to the formation of IPP and DMAPP. LytB was epitope-tagged to facilitate its removal from cell extracts. PPC substrate stimulation of LytB deficient extracts was similar to that in wild-type, indicating that LytB is not involved in isoprenoid production via this route. However, a Sll1556 protein is clearly required for PPC substrate stimulated synthesis. Inactivation of the sll1556 gene abolishes isoprenoid formation in vitro, and biosynthesis is restored with the purified protein. The collective results suggest that isoprenoids synthesized from PPC substrates are made via a PPC shunt, rather than through the linear MEP pathway in Synechocystis PCC 6803. Under environmental conditions such as optimal light, an added PPC substrate pool for enhanced isoprenoid production could provide a clear competitive advantage to a photosynthetic organism. Possible interaction of the PPC shunt with the MEP pathway is being explored. *PPCI (Pentose Phosphate Cycle stimulated Isoprenoid biosynthesis) is suggested as the abbreviation for this newly defined shunt.