In wild-type C. elegans , touch cell fate is restricted to six cells. In
egl-44 and
egl-46 mutants, two other neurons, the FLP cells, express touch receptor-like features.
egl-44 and
egl-46 were first identified by egg-laying defective mutants with defects in the development of the HSN neurons (Desai et al., Nature 336: 638-646, 1988; Desai and Horvitz, Genetics 121: 703-721, 1989). Thus,
egl-44 and
egl-46 mutations affect several different neurons. We cloned
egl-44 and
egl-46 by transformation rescue, identified the mutations in the existing mutant strains, and recovered full-length cDNAs.
egl-44 produces a 2.1 kb mRNA and
egl-46 produces a 1.6 kb mRNA.
egl-44 encodes a member of the TEF (transcription enhancer factor) family. TEF proteins, found from yeast to human, are involved in a variety of developmental processes. Like other family members, EGL-44 protein contains a specific DNA-binding domain (the TEA/ATTS domain) and a transcriptional regulation domain.
egl-44::gfp reporter constructs are expressed in a few neurons in the head and tail and in some intestinal cells. Some staining in the region near FLP suggests that they may express
egl-44 . Although all existing mutations are missense changes, RNA i suggested that their phenotype is the null phenotype.
egl-46 encodes a protein containing some Zn-finger motifs similar to those in the IA-1 protein, a protein expressed in many human neuroendocrine tumor and small cell lung cancer cell.
egl-46::gfp fusions are expressed in the Q lineages, which undergo extra division in
egl-46 mutant. Both the EGL-46 protein and the 3' end of its mRNA contain sequences that may target the gene products to rapid degradation. Consistent with these sequence elements, expression of
egl-46::gfp fusions is transient in HSN, FLP and ventral nerve cord neurons. Since both
egl-44 and
egl-46 are expressed in the FLP cells, yet mutation in either one changes FLP cell fate, both genes are needed for repression of touch cell characteristic. Indeed, an
egl-46 promoter fusion with gfp is expressed in the ALM, AVM and PVM cells. Moreover,
egl-46 expressed from
mec-7 promoter has no detectable effect on touch cell function. We are currently examining the effects of coexpression of these two genes in the touch cells.