In wild-type C. elegans only six cells develop as receptors for gentle touch. 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 . Thus,
egl-44 and
egl-46 mutations affect the differentiation of several different neurons.
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 different cell types, including neurons, hypodermal cells and some intestinal cells The product of the
egl-46 gene and similar proteins in Drosophila, mouse and human, define a new family of Zn-finger proteins, which are expressed in neural precursor cells.
egl-46::gfp fusions are expressed in the Q lineages, which undergo extra division in
egl-46 mutants. Both
egl-44 and
egl-46 are expressed in FLP neurons and HSN neurons (as well as other cells), but only in the former cells is
egl-44 needed for full expression of
egl-46. 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, touch cells and ventral nerve cord neurons. Since expression of both genes or
egl-44 in wild-type touch cells causes touch insensitivity and loss of touch cell-specific gene expression, but
egl-46 in wild-type and
egl-44 in a
egl-46 mutant do not significantly affect the touch cells, both genes are required to repress touch cell fate. The sequences of these genes and their nuclear location suggest that these genes repress transcription of touch cell characteristics in the FLP cells.