PHA-4 IS CE-FKH-1, A FORK HEAD HNF-3-ALPHA,BETA,GAMMA HOMOLOG THAT FUNCTIONS IN ORGANOGENESIS OF THE C. ELEGANS PHARYNX/

The C. elegans Ce-fkh-1 gene has been cloned on the basis of its seque nce similarity to the winged-helix DNA binding domain of the Drosophil a fork head and mammalian HNF3 alpha,beta,gamma genes, and mutations i n the zygotically active pha-1 gene have been shown to block formation of the pharynx (and rectum) at an early stage in embryogenesis. In th e present paper, we show that Ce-fkh-1 and pha-4 are the same gene. We show that PHA-4 protein is present in nuclei of essentially all phary ngeal cells, of all five cell types. PHA-4 protein first appears close to the point at which a cell lineage will produce only pharyngeal cel ls, independently of cell type. We show that PHA-4 binds directly to a 'pan-pharyngeal enhancer element' previously identified in the promot er of the pharyngeal myosin myo-2 gene; in transgenic embryos, ectopic PHA-4 activates ectopic myo-2 expression. We also show that ectopic P HA-4 can activate ectopic expression of the ceh-22 gene, a pharyngeal- specific NK-2-type homeodomain protein previously shown to bind a musc le-specific enhancer near the PHA-4 binding site in the myo-2 promoter . We propose that it is the combination of pha-4 and regulatory molecu les such as ceh-22 that produces the specific gene expression patterns during pharynx development. Overall, pha-4 can be described as an 'or gan identity factor', completely necessary for organ formation, presen t in all cells of the organ from the earliest stages, capable of integ rating upstream developmental pathways (in this case, the two distinct pathways that produce the anterior and posterior pharynx) and partici pating directly in the transcriptional regulation of organ specific ge nes. Finally, we note that the distribution of PHA-4 protein in C. ele gans embryos is remarkably similar to the distribution of the fork hea d protein in Drosophila embryos: high levels in the foregut/pharynx an d hindgut/rectum; low levels in the gut proper. Moreover, we show that pha-4 expression in the C. elegans gut is regulated by elt-2, a C. el egans gut-specific GATA-factor and possible homolog of the Drosophila gene serpent, which influences fork head expression in the fly gut. Ov erall, our results provide evidence for a highly conserved pathway reg ulating formation of the digestive tract in all (triploblastic) metazo a.