coli, the folX deletant lacked tetrahydromonapterin. The Δ tyrA strain was, as expected, prototrophic for tyrosine, whereas the Δ tyrA Δ folX strain was auxotrophic. aeruginosa by deleting tyrA (making PhhA the sole source of tyrosine) and folX. Purified FolM showed high, NADPH-dependent dihydromonapterin reductase activity. The folX deletion selectively eliminated tetrahydromonapterin production, which far exceeded folate production. This rescue was abrogated by deleting folX or folM and restored by expressing the deleted gene from a plasmid. aeruginosa PhhA plus the recycling enzyme pterin 4a-carbinolamine dehydratase, PhhB, rescues tyrosine auxotrophy. coli, which lacks PhhA and in which the expression of P. The roles of FolX and FolM were tested experimentally first in E. folM encodes an unusual short-chain dehydrogenase/reductase known to have dihydrofolate and dihydrobiopterin reductase activity. folX encodes dihydroneopterin triphosphate epimerase, which interconverts dihydroneopterin triphosphate and dihydromonapterin triphosphate. A comparative genomics analysis implicated the enigmatic folX and folM genes in tetrahydromonapterin synthesis via their phyletic distribution and chromosomal clustering patterns. Tetrahydromonapterin is a major pterin in Escherichia coli and is hypothesized to be the cofactor for phenylalanine hydroxylase (PhhA) in Pseudomonas aeruginosa, but neither its biosynthetic origin nor its cofactor role has been clearly demonstrated.
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