Lipooligosaccharide (LOS) is a critical virulence factor of Neisseria meningitidis. A Tn916 insertion mutant, designated 469, was found to exhibit a markedly truncated LOS of 2.9 kDa when compared by Tricine/SDS-PAGE to the parental LOS (4.6 kDa). Electrospray mass spectrometry analysis of 469 LOS revealed that it consisted of the deep rough, heptose-deficient structure, Kdo(2)-lipid A. Sequencing of chromosomal DNA flanking the Tn916 insertion in mutant 469 revealed that the transposon had inserted into an ORF predicted to encode a 187 aa protein with sequence homology to the histidinol-phosphate phosphatase domain of Escherichia coli HisB and to a family of genes of unknown function. The gene, designated gmhX, is part of a polycistronic operon (ice-2) containing two other genes, nlaB and orfC. nlaB encodes a lysophosphatidic-acid acyltransferase and orfC is predicted to encode a N-acetyltransferase. Specific polar and non-polar gmhX mutations in the parental strain, NMB, exhibited the truncated LOS structure of mutant 469, and repair of gmhX mutants by homologous recombination with the wild-type gmhX restored the LOS parental phenotype. GmhX mutants demonstrated increased sensitivity to polymyxin B. GmhX mutants and other Kdo(2)-lipid A mutants also demonstrated increased sensitivity to killing by normal human serum but were not as sensitive as inner-core mutants containing heptose. In the genomes of Helicobacter pylori and Synechocystis, gmhX homologues are associated with heptose biosynthesis genes; however, in N. meningitidis, gmhX was found in a location distinct from that of gmhA, rfaD, rfaE, aut and rfaC. GmhX is a novel enzyme required for the incorporation of L-glycero-D-manno-heptose into meningococcal LOS, and is a candidate for the 2-D-glycero-manno-heptose phosphatase of the heptose biosynthesis pathway.