Microbacterium nematophilum is a recently-identified bacterial pathogen of Caenorhabditis elegans (Hodgkin et al. , 2000). These bacteria adhere to the rectal and post-anal cuticle of the worm and induce swelling of the underlying hypodermal tissue. Screens for mutant worms resistant to infection (bacterially un-swollen; Bus) have been carried out using a mutator strain (
mut-7 ) in which germline transposition of Tc1 and other transposons is activated. One strain obtained from this screen, was initially named
bus-11 , and has a complex phenotype. In addition to exhibiting the Bus phenotype, the hermaphrodites are egg-laying defective, uncoordinated and constipated. The males have tail developmental abnormalities. Probing of a Southern blot of genomic DNA isolated from
bus-11 worms with a Tc1-specific probe demonstrated the presence of a novel Tc1 insertion that co-segregated with the
bus-11 phenotype. Transposon insertion display was used to sequence the genomic region flanking this insertion. The sequence obtained lay in the region to which
bus-11 had been mapped (LGIII between
unc-32 and
lon-1 ) and indicated that the insertion was within the open reading frame of two genes encoding a prenyltransferase (B0280.1) and a KH-domain-containing protein (B0280.11). Preliminary RNA interference experiments to knock out both these genes failed to emulate the
bus-11 phenotype suggesting that this Tc1 insertion may be closely linked to
bus-11 but not causative of the phenotype. Inspection of the genetic map in this region identified the
egl-5 gene as an alternative candidate on account of the striking phenotypic similarities between
egl-5 and
bus-11 mutant strains.
egl-5 is a Hox gene involved in the patterning of posterior structures.
egl-5 mutants were duly tested for response to the pathogen. Of four mutant strains tested, three (
n1066 ,
n945 e1239 ,
n988 e499 ) demonstrated a Bus phenotype suggesting that the resistance mutation in our strain is indeed in the
egl-5 gene. The fourth strain tested (
n1439 ) contains a weak
egl-5 mutant allele. While egg-laying defective, these worms are not uncoordinated nor do males show the tail abnormalities of other
egl-5 mutant strains (Chisholm, 1991). Interestingly, these animals showed marked swelling of the anal region on exposure to the pathogen. This allele may thus provide a tool for the dissection of
egl-5 function and elucidation of its role in susceptibility to M. nematophilum infection. We are now investigating the response to the pathogen of other
egl-5 mutants and mutants of additional genes that have been implicated either in the regulation of
egl-5 function or as
egl-5 targets. Hodgkin, J., Kuwabara, P.E. and Corneliussen, B. (2000) A novel bacterial pathogen, Microbacterium nematophilum , induces morphological change in the nematode C. elegans . Current Biology 10:1615-1618 Chisholm, A. (1991) Control of cell fate in the tail region of C. elegans by the gene
egl-5 . Development 111:921-932