Identification of a thiamin-dependent synthase in <i>Escherichia coli</i> required for the formation of the 1-deoxy- <scp>d</scp> -xylulose 5-phosphate precursor to isoprenoids, thiamin, and pyridoxol

Abstract

In Escherichia coli , 1-deoxy- d -xylulose (or its 5-phosphate, DXP) is the biosynthetic precursor to isopentenyl diphosphate [Broers, S. T. J. (1994) Dissertation (Eidgenössische Technische Hochschule, Zürich)], thiamin, and pyridoxol [Himmeldirk, K., Kennedy, I. A., Hill, R. E., Sayer, B. G. &amp; Spenser, I. D. (1996) Chem. Commun . 1187–1188]. Here we show that an open reading frame at 9 min on the chromosomal map of E. coli encodes an enzyme ( d eoxy x ylulose-5- p hosphate synthase, DXP synthase) that catalyzes a thiamin diphosphate-dependent acyloin condensation reaction between C atoms 2 and 3 of pyruvate and glyceraldehyde 3-phosphate to yield DXP. We have cloned and overexpressed the gene ( dxs ), and the enzyme was purified 17-fold to a specific activity of 0.85 unit/mg of protein. The reaction catalyzed by DXP synthase yielded exclusively DXP, which was characterized by 1 H and 31 P NMR spectroscopy. Although DXP synthase of E. coli shows sequence similarity to both transketolases and the E1 subunit of pyruvate dehydrogenase, it is a member of a distinct protein family, and putative DXP synthase sequences appear to be widespread in bacteria and plant chloroplasts.