Cilia are an important cellular differentiation of sensory neurons for receiving information from the environment. 60 of the 302 C. elegans neurons have ciliated endings. Mutations in a large number of genes have been identified that affect the structure of these sensory cilia. Of these,
daf-19 mutations are unique in completely lacking all sensory cilia. Aside from the absence of sensory cilia, the neurons seem to be morphologically normal. We previously reported that
daf-19 encodes an RFX-type transcription factor that is expressed in ciliated sensory neurons.We also showed that DAF-19 regulates via its target site in promoter regions, the x-box, a number of effector genes involved in sensory cilium formation. These effector genes are expressed in essentially all ciliated sensory neurons and, when mutated, cause morphological defects in sensory cilia (e.g.
che-2,
osm-1,
osm-6). Our results strongly suggest that DAF-19 regulates the differentiation of sensory cilia by activating the transcription of a battery of genes whose products form the sensory cilium. Cilia and flagella are structurally very similar subcellular organelles. Work done on the unicellular, flagellated green alga Chlamydomonas lead to an estimate of around 300 different protein components required for structure and function of flagella. We expect a similar number for cilium structure and function. We aim to initiate the identification of all ciliary components by analyzing genes that have an appropriately spaced x-box promoter element: so called xbx genes, hypothesized to be regulated by
daf-19. In a first search through the C. elegans genome sequence, using an algorithm designed to find promoter elements, we uncovered more than 200 candidate xbx genes, several of which had C. briggsae homologues that also had an appropriately positioned x-box promoter element. Initial expression analyses of a subgroup of these xbx genes showed that, indeed, some of them were specifically expressed in ciliated sensory neurons in a
daf-19 dependent manner. In another approach, using oligo-nucleotide arrays representing nearly all genes of C. elegans (1), we compared the gene expression profile of wild type and
daf-19 mutants. A small group of genes was reproducibly down-regulated in a
daf-19 background, among them known cilium-structure genes and several xbx genes. In combination with previous data about sensory cilia, these approaches have the potential to reveal all the genes required for sensory cilium function and structure. (1) in collaboration with Allan Jones (Rosetta Inpharmatics, Kirkland, WA, U.S.A.)