CLONING AND NUCLEOTIDE-SEQUENCE OF A FLAGELLIN ENCODING GENETIC-LOCUSFROM XENORHABDUS-NEMATOPHILUS - PHASE VARIATION LEADS TO DIFFERENTIALTRANSCRIPTION OF 2 FLAGELLAR GENES (FLICD)

The insect-pathogenic bacterium Xenorhabdus undergoes spontaneous phas e variation involving a large number of phenotypes. Our previous study indicated that phase I variants were motile, whereas phase II variant s of X. nematophilus F1 were non-flagellated cells which did not synth esize flagellin [Givaudan, A., Baghdiguian, S., Lanois, A. and Boemare , N. (1995) Appl. Environ. Microbiol. 61, 1408-1413]. In order to appr oach the study of the flagellar switching, a locus containing two ORFs from X. nematophilus F1 (phase I) was identified by using functional complementation of flagellin-negative E. coli. The sequence analysis r evealed that the first ORF corresponds to the fliC gene coding for fla gellin, and showed a high degree of homology between the N-terminal an d C-terminal of Xenorhabdus FliC and flagellins from other bacteria. T he second identified ORF in the opposite orientation encodes a homolog ue of the enterobacterial hook-associated protein 2, FGD. Both Xenorha bdus fliCD genes were required for the entire restoration of E. coli m otility. A sequence highly homologous to the sigma(28) consensus promo ter was identified upstream from the coding sequences from both genes. The structure of the fliC gene and its surrounding region was shown t o be the same in both phase variants, but Northern blot analysis revea led that fliC and fliD were, respectively, not and weakly transcribed in phase II variants. In addition, complementation experiments showed that motility and flagellin synthesis of phase II cannot be recovered by placing in trans fliCD genes from phase I. These latter results sug gest that a gene(s) higher in the transcriptional hierarchy of the fla gellar regulon is switched off in Xenorhabdus phase II variants.