Epithelial cells in all organisms are defined by apico-basal polarity and the possession of specific cell-cell junctions. There are generally two types of epithelial junctions: adherens junctions, which are involved in cell-cell adhesion, and tight (vertebrate) or septate (invertebrate) junctions which maintain apico-basal polarity. We are interested in epithelial cell function in C. elegansand are investigating homologues of proteins involved in epithelial polarity in other species, especially those associated with tight or septate junctions. These junctions are apparently absent from most epithelial cells in C. elegansand yet these cells possess apico-basal polarity. We performed a RNA interference (RNAi) screen of 18 of these homologues and decided to focus on one predicted gene, C25F6.2, as it results in an embryonic lethal RNAi phenotype with arrest during morphogenesis. We have named this predicted gene dlg-1 as it is the C. elegans orthologue of the gene encoding the Drosophila Lethal Discs large (Dlg) protein. The dlg-1(RNAi) phenotype is 100% embryonic lethality at around the 2-to 2.5-fold stage of morphogenesis. These embryos all have leaking cells or rupture of the epidermis and are not paralysed. Characterisation of dlg-1(RNAi) embryos using immunofluorescence staining revealed that MH27 (which recognises the epithelial adherens junction protein JAM-1) staining is abnormal with a punctate, discontinuous pattern in the epidermis, pharynx and intestine. Basal (MH4, MH5 and MH46 antibodies) and apical (anti-PKC-3 antibody) epithelial markers are normal, and a muscle specific antibody, NE8/4C6.3, also shows a wild type staining pattern. A full length dlg-1::gfp was constructed to determine the DLG-1 expression pattern. DLG-1::GFP has an identical temporal and spatial localisation to MH27: adherens junctions of the hypodermis, pharynx, intestine, excretory pore and amphid support cells in embryos and larvae and, additionally, the spermatheca, uterus and vulva in adult hermaphrodites. It is striking that the orthologue of a septate junction-associated protein in Drosophila known to be essential for cell polarity is localised to adherens junctions in C. elegans epithelial cells. This raises the possibility that C. elegans adherens junctions may have other roles in addition to that of cell-cell adhesion. Mutants of the let-413 gene (see abstract by R.Legouis et.al.) arrest before or around the 2-fold stage of morphogenesis and either rupture completely or extrude some cells. MH27 staining is also completely abnormal in these embryos showing a similar but more severe discontinuous pattern to that in the dlg-1(RNAi) embryos. Due to the similarity of the dlg-1(RNAi) and let-413 mutant phenotypes, we decided to look for interactions between these genes. RNAi against the let-413 gene in dlg-1::gfp expressing embryos mislocalises the DLG-1::GFP producing a punctate, discontinuous pattern with varying thickness of the DLG-1::GFP lines denoting adherens junctions. This abnormal dlg-1::gfp pattern is identical to MH27 staining in these embryos. However, RNAi against dlg-1 in embryos expressing a full length let-413::gfp does not affect the LET-413::GFP which remains localised to the basolateral membrane. In conclusion, DLG-1 and LET-413 appear to be involved in the assembly of adherens junctions and are necessary for the completion of embryonic morphogenesis. Since dlg-1(RNAi) embryos are less severely affected than let-413 mutants in many respects, this would suggest that dlg-1 is one of the downstream targets of let-413. The investigation of protein-protein interactions between DLG-1 and LET-413 is underway and should elucidate the pathway(s) involved in adherens junction assembly and their role in morphogenesis.