Characterization of a lipoprotein, NilC, required by Xenorhabdus nematophila for mutualism with its nematode host

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Abstract
Summary: Xenorhabdus nematophila is a γ‐proteobacterial mutualist of an insect‐pathogenic nematode, Steinernema carpocapsae. X. nematophila requires nilC, a gene predicted to encode an outer membrane lipoprotein of unknown function, for colonization of its nematode host. Characterization of NilC, described here, demonstrated it is a 28 kDa lipoprotein directed to the periplasm by an N‐terminal signal sequence. Lipidation and processing of NilC occurs by a mechanism that is conserved in proteobacteria. This work also showed NilC is membrane associated and oriented towards the periplasm of X. nematophila and is produced as an outer membrane‐associated protein when expressed in Escherichia coli. Expression analyses revealed that nilC transcription is directly or indirectly repressed by Lrp, and this regulatory link may explain the nematode mutualism defect of a previously identified lrp::Tn5 mutant. An lrp::Tn5 mutant produces an additional nilC transcript, not observed in wild‐type cells growing in vitro, and produces ≈ 75‐fold more nilC than wild‐type cells in late stationary phase. These fundamental characterizations of nilC expression and nilC localization and processing events have provided firm bases for understanding the role of this colonization factor in the X. nematophila/S. carpocapsae microbe–host interaction.