During C. elegans development, many blast cells generate both neurons and hypodermal cells. What factors make neurons and hypodermal cells dlfferent? To address this question. we are studying the mutatlon Ifn-26tnl56), whlch transforms the normal hypodermal cell fate of the Pn.p cells to a neuronal fate slmllar to that of thelr llneal slsters. the Pn.a cells (see Labouesse and Horvitz, 1990, WBG 11 (4), p. 70). We have cloned the gene Ifn-26 by mapplng deflclency endpolnts and by germilne transformatlon. A 9 kb reglon of cosmid C18C9 rescues
nl56. This region encodes at least four transcripts: three RNAs (1.4 kb, 1.45 kb and 1.5 kb) arise via altemative splicing near thelr 3' ends; a fourth RNA, 2.2 kb long, overlaps in its 5' untranslated region with the 3' untranslated region of the other three transcripts. The two sets of transcripts encode proteins with similar structure which are likely to be transcription factors. Both sets of proteins contain: a serine/threonine-rich amino-terminal region, a glutamine-rich central region, two copies of a cysteine-histidine motif related to but distinct from the zinc fingers present in the TFIIIA transcription factor, and an acid-rich carboxy-terminal region; ln the first set of transcripts, it is the acid-rich that is modified by alternative splicing. By injecting constructs carrying a frameshift mutation either in the first set of transcripts or in the 2.2 kb transcript, we have found that the latter ls responsible for the
nl56- resculng activlty. We do not know the function of the other set of RNAs. Sequence analysis shows that the mutation
nl56 changes a conserved leucine residue in the first zinc finger of the 2.2 kb RNA to phenylalanine. A Ifn-26/1acZ gene fuslon has been made by insertlng an 8 kb DNA fragment (1 kb of the 2.2 kb RNA coding sequence and 7 kb of upstream sequence) lnto Andy Fire's vector pPD22.04. Thls construct has been injected along wlth the marker
rol-6(sulO06J lnto the gonad of N2 hermaphrodltes. In transformed llnes with an extrachromosomal array, stalning with X-gal ls observed ln most hypodermal cells (
hyp4 through hypll) and most ectoblast cells (the P cells, B, F, Y, U). starting in embryogenesis at the time these cells are bom. In addition, we have begun a preliminary characterization of
nl56/mnDf88 animals ( mnDf88 is a deflclency which takes out the Ifn-26 locus). Such anlmals dle as Ll larvae with hypodermal defects: the tail is grossly deformed, the cuticle is detached. alae are usually absent, some anlmals have no anus, others dlsplay protruslons (as lf the body was not encompassed by the hypodermis). The Ifn-26/1acZ staining pattern, the presence of transcription factors motifs in
lin-26 proteins and the terminal phenotype of
nl56/mnDf88 animals together suggest that Ifn-26 could be a regulator of hypodermal difrerentiation.