The gene
srf-2 was initially identified in screens for mutants with altered surface antigenicity (S. Politz et al., 1990). Subsequently it has been shown to affect susceptibility to several bacterial pathogens. Srf-2 mutants are resistant to the formation of Yersinia biofilms on the head (the Bah, or Biofilm Absent on Head phenotype) and to tail-swelling induced by Microbacterium nematophilum (the Bus, or Bacterially Un-Swollen phenotype). Conversely Srf-2 mutants are hypersensitive to Leucobacter Verde1.
Molecular cloning of
srf-2 reveals that it encodes a predicted secreted protein with a DUF23 domain, belonging to a family of 61 related genes in C. elegans. Most of these DUF23 genes have no currently known biological function, but one of them is the gene
bah-1, which affects adhesion of Yersinia (K. Drace et al., 2009). Bah-1 mutants, however, are resistant only to Yersinia, unlike Srf-2 mutants which are resistant to multiple pathogens. Double mutants (
srf-2 bah-1) resemble
srf-2 single mutants. The DUF23 domain is currently of uncertain biochemical function, but may be associated with glycosylation. The sequence alterations in 9
srf-2 mutations (both EMS and mutator-induced) have been determined. These include transposon insertions and lesions affecting a splice site and most exons of the coding region. All are likely to result in severe or complete loss-of-function, and have similar phenotypes.
Transgenic reporter constructs indicate that the major site of expression for
srf-2 is in the seam cells, like many other surface-affecting genes including
bah-1. Effects on the cuticular surface have been examined by scanning EM. This reveals differences in several aspects of cuticle morphology in Srf-2 mutants, including the annuli, furrows and alae, which appear less defined than in wildtype. Movement of Srf-2 mutants is also slightly abnormal, with a weak Skiddy phenotype.
Srf-2 mutants are unable to grow on bacterial lawns containing the surface-adherent bacterial pathogen Leucobacter Verde1, probably as a result of lethal cuticle damage. Suppressors or revertants of
srf-2 can therefore be efficiently selected by mutagenizing
srf-2 populations and exposing them to Verde1 lawns. By this means, several suppressed lines have been obtained. Further analysis of these suppressors is in progress.