In all organisms studied so far, isoprenoids derive from the common isoprene units, isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP). In mammals and in fungi, IPP and DMAPP are formed exclusively by the mevalonate pathway (
11). In contrast, many eubacteria (including
Escherichia coli), algae, and the plastids of higher plants synthesize IPP and DMAPP by the 2-
C-methyl-
d-erythritol 4-phosphate (MEP) pathway (
9,
34). The MEP pathway was also identified in a plastid-like organelle of malaria parasites (
15). Since the MEP pathway is absent in humans, it has been validated as a drug target for the treatment of both bacterial and parasitic infections (
15,
29).
The pathway initiates with the formation of 1-deoxy-
d-xylulose 5-phosphate (DOXP) by condensation of pyruvate and
d-glyceraldehyde 3-phosphate catalyzed by the DOXP synthase (Dxs) (
1,
6,
20,
22,
24,
25,
35,
38). DOXP is then converted by the DOXP reductoisomerase (Dxr) into MEP (Fig. ) (
1,
12,
21,
28,
30,
36,
40). According to recent findings, the enzymes encoded by the genes
ygbP, ychB, and
ygbB are able to catalyze the formation of 2-
C-methyl-
d-erythritol 2,4-cyclodiphosphate, with 4-diphosphocytidyl-2-
C-methyl-
d-erythritol as an intermediate (
14,
18,
19,
26,
33,
39). The subsequent biochemical steps of the MEP pathway are still unknown.
Recent evidence (
2,
7,
27,
32) indicates that the MEP pathway produces IPP and DMAPP separately after a branching point downstream from MEP. In addition, IPP and DMAPP can be interconverted in
E. coli by the IPP isomerase (Ipi); however, this enzyme is not essential for survival and consequently absent in various other bacteria using the MEP pathway, as shown for
Synechocystis (
10,
13).
In a search for other genes involved in the MEP pathway, it was demonstrated that an enzyme encoded by the
lytB gene catalyzes an essential step at, or subsequent to, the point at which the MEP pathway branches to form IPP and DMAPP (
8). Using genomic databases, a pattern of occurrence identical to that of the described genes of the MEP pathway was identified for the genes
lytB and
gcpE (
8). Therefore,
gcpE must be considered a candidate for another gene of the MEP pathway. In former work,
gcpE was shown to be essential for the growth of bacteria, but no clear function could be attributed to it (
4).
In this work, we demonstrate that gcpE is essentially involved in the MEP pathway. In a first step, E. coli cells were genetically engineered to utilize exogenously provided mevalonate for isoprenoid biosynthesis by introduction of three genes of the yeast mevalonate pathway (Fig. ). In a second step, the chromosomal gcpE gene of the engineered cells was deleted. The resulting mutants were viable only when the culture medium was supplemented with mevalonate, similar to dxr-deficient bacteria serving as controls. The ability to grow in the absence of mevalonate could be restored by transformation with a plasmid containing the gcpE gene.